diff --git a/packages/react-devtools-scheduling-profiler/src/constants.js b/packages/react-devtools-scheduling-profiler/src/constants.js
index e5f9336d73e78..2e463ae632d95 100644
--- a/packages/react-devtools-scheduling-profiler/src/constants.js
+++ b/packages/react-devtools-scheduling-profiler/src/constants.js
@@ -11,6 +11,5 @@ export {
   COMFORTABLE_LINE_HEIGHT,
   COMPACT_LINE_HEIGHT,
 } from 'react-devtools-shared/src/constants.js';
-import {TotalLanes} from 'react-reconciler/src/ReactFiberLane.new';
 
-export const REACT_TOTAL_NUM_LANES = TotalLanes;
+export const REACT_TOTAL_NUM_LANES = 31;
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js b/packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js
index 67d1a29f0ab03..2ad221d5fbcb6 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js
@@ -380,6 +380,7 @@ describe('ReactDOMServerPartialHydration', () => {
     resolve();
     await promise;
     Scheduler.unstable_flushAll();
+    await null;
     jest.runAllTimers();
 
     // We should now have hydrated with a ref on the existing span.
diff --git a/packages/react-dom/src/__tests__/ReactServerRenderingHydration-test.js b/packages/react-dom/src/__tests__/ReactServerRenderingHydration-test.js
index fea9d79b4735d..a361e55a7f8fc 100644
--- a/packages/react-dom/src/__tests__/ReactServerRenderingHydration-test.js
+++ b/packages/react-dom/src/__tests__/ReactServerRenderingHydration-test.js
@@ -488,6 +488,7 @@ describe('ReactDOMServerHydration', () => {
     jest.runAllTimers();
     await Promise.resolve();
     Scheduler.unstable_flushAll();
+    await null;
     expect(element.textContent).toBe('Hello world');
   });
 
diff --git a/packages/react-is/index.experimental.js b/packages/react-is/index.experimental.js
new file mode 100644
index 0000000000000..560283842b594
--- /dev/null
+++ b/packages/react-is/index.experimental.js
@@ -0,0 +1,41 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+'use strict';
+
+export {
+  isValidElementType,
+  typeOf,
+  ContextConsumer,
+  ContextProvider,
+  Element,
+  ForwardRef,
+  Fragment,
+  Lazy,
+  Memo,
+  Portal,
+  Profiler,
+  StrictMode,
+  Suspense,
+  unstable_SuspenseList,
+  isAsyncMode,
+  isConcurrentMode,
+  isContextConsumer,
+  isContextProvider,
+  isElement,
+  isForwardRef,
+  isFragment,
+  isLazy,
+  isMemo,
+  isPortal,
+  isProfiler,
+  isStrictMode,
+  isSuspense,
+  unstable_isSuspenseList,
+} from './src/ReactIs';
diff --git a/packages/react-is/index.stable.js b/packages/react-is/index.stable.js
new file mode 100644
index 0000000000000..ad64178a1c537
--- /dev/null
+++ b/packages/react-is/index.stable.js
@@ -0,0 +1,39 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+'use strict';
+
+export {
+  isValidElementType,
+  typeOf,
+  ContextConsumer,
+  ContextProvider,
+  Element,
+  ForwardRef,
+  Fragment,
+  Lazy,
+  Memo,
+  Portal,
+  Profiler,
+  StrictMode,
+  Suspense,
+  isAsyncMode,
+  isConcurrentMode,
+  isContextConsumer,
+  isContextProvider,
+  isElement,
+  isForwardRef,
+  isFragment,
+  isLazy,
+  isMemo,
+  isPortal,
+  isProfiler,
+  isStrictMode,
+  isSuspense,
+} from './src/ReactIs';
diff --git a/packages/react-is/src/ReactIs.js b/packages/react-is/src/ReactIs.js
index 2f132ba5de9a0..1958043bb2243 100644
--- a/packages/react-is/src/ReactIs.js
+++ b/packages/react-is/src/ReactIs.js
@@ -72,6 +72,7 @@ export const Portal = REACT_PORTAL_TYPE;
 export const Profiler = REACT_PROFILER_TYPE;
 export const StrictMode = REACT_STRICT_MODE_TYPE;
 export const Suspense = REACT_SUSPENSE_TYPE;
+export const unstable_SuspenseList = REACT_SUSPENSE_LIST_TYPE;
 
 export {isValidElementType};
 
@@ -142,3 +143,6 @@ export function isStrictMode(object: any) {
 export function isSuspense(object: any) {
   return typeOf(object) === REACT_SUSPENSE_TYPE;
 }
+export function unstable_isSuspenseList(object: any) {
+  return typeOf(object) === REACT_SUSPENSE_LIST_TYPE;
+}
diff --git a/packages/react-is/src/__tests__/ReactIs-test.js b/packages/react-is/src/__tests__/ReactIs-test.js
index cddd479c4344e..e054cdbc4d485 100644
--- a/packages/react-is/src/__tests__/ReactIs-test.js
+++ b/packages/react-is/src/__tests__/ReactIs-test.js
@@ -186,6 +186,24 @@ describe('ReactIs', () => {
     expect(ReactIs.isSuspense(<div />)).toBe(false);
   });
 
+  // @gate experimental
+  it('should identify suspense list', () => {
+    expect(ReactIs.isValidElementType(React.unstable_SuspenseList)).toBe(true);
+    expect(ReactIs.typeOf(<React.unstable_SuspenseList />)).toBe(
+      ReactIs.unstable_SuspenseList,
+    );
+    expect(
+      ReactIs.unstable_isSuspenseList(<React.unstable_SuspenseList />),
+    ).toBe(true);
+    expect(
+      ReactIs.unstable_isSuspenseList({type: ReactIs.unstable_SuspenseList}),
+    ).toBe(false);
+    expect(ReactIs.unstable_isSuspenseList('React.unstable_SuspenseList')).toBe(
+      false,
+    );
+    expect(ReactIs.unstable_isSuspenseList(<div />)).toBe(false);
+  });
+
   it('should identify profile root', () => {
     expect(ReactIs.isValidElementType(React.Profiler)).toBe(true);
     expect(
diff --git a/packages/react-native-renderer/src/ReactFabric.js b/packages/react-native-renderer/src/ReactFabric.js
index 03ad183c37df4..9761124e7da65 100644
--- a/packages/react-native-renderer/src/ReactFabric.js
+++ b/packages/react-native-renderer/src/ReactFabric.js
@@ -9,7 +9,7 @@
 
 import type {HostComponent} from './ReactNativeTypes';
 import type {ReactNodeList} from 'shared/ReactTypes';
-import type {ElementRef} from 'react';
+import type {ElementRef, Element, ElementType} from 'react';
 
 import './ReactFabricInjection';
 
@@ -47,8 +47,8 @@ import getComponentName from 'shared/getComponentName';
 
 const ReactCurrentOwner = ReactSharedInternals.ReactCurrentOwner;
 
-function findHostInstance_DEPRECATED(
-  componentOrHandle: any,
+function findHostInstance_DEPRECATED<TElementType: ElementType>(
+  componentOrHandle: ?(ElementRef<TElementType> | number),
 ): ?ElementRef<HostComponent<mixed>> {
   if (__DEV__) {
     const owner = ReactCurrentOwner.current;
@@ -70,10 +70,14 @@ function findHostInstance_DEPRECATED(
   if (componentOrHandle == null) {
     return null;
   }
+  // $FlowIssue Flow has hardcoded values for React DOM that don't work with RN
   if (componentOrHandle._nativeTag) {
+    // $FlowIssue Flow has hardcoded values for React DOM that don't work with RN
     return componentOrHandle;
   }
+  // $FlowIssue Flow has hardcoded values for React DOM that don't work with RN
   if (componentOrHandle.canonical && componentOrHandle.canonical._nativeTag) {
+    // $FlowIssue Flow has hardcoded values for React DOM that don't work with RN
     return componentOrHandle.canonical;
   }
   let hostInstance;
@@ -194,10 +198,10 @@ function sendAccessibilityEvent(handle: any, eventType: string) {
 }
 
 function render(
-  element: React$Element<any>,
-  containerTag: any,
-  callback: ?Function,
-) {
+  element: Element<ElementType>,
+  containerTag: number,
+  callback: ?() => void,
+): ?ElementRef<ElementType> {
   let root = roots.get(containerTag);
 
   if (!root) {
@@ -208,6 +212,7 @@ function render(
   }
   updateContainer(element, root, null, callback);
 
+  // $FlowIssue Flow has hardcoded values for React DOM that don't work with RN
   return getPublicRootInstance(root);
 }
 
diff --git a/packages/react-native-renderer/src/ReactFabricHostConfig.js b/packages/react-native-renderer/src/ReactFabricHostConfig.js
index e06652048e3da..85eb2a741ffac 100644
--- a/packages/react-native-renderer/src/ReactFabricHostConfig.js
+++ b/packages/react-native-renderer/src/ReactFabricHostConfig.js
@@ -14,7 +14,7 @@ import type {
   MeasureLayoutOnSuccessCallback,
   MeasureOnSuccessCallback,
   NativeMethods,
-  ReactNativeBaseComponentViewConfig,
+  ViewConfig,
   TouchedViewDataAtPoint,
 } from './ReactNativeTypes';
 
@@ -111,13 +111,13 @@ if (registerEventHandler) {
  */
 class ReactFabricHostComponent {
   _nativeTag: number;
-  viewConfig: ReactNativeBaseComponentViewConfig<>;
+  viewConfig: ViewConfig;
   currentProps: Props;
   _internalInstanceHandle: Object;
 
   constructor(
     tag: number,
-    viewConfig: ReactNativeBaseComponentViewConfig<>,
+    viewConfig: ViewConfig,
     props: Props,
     internalInstanceHandle: Object,
   ) {
diff --git a/packages/react-native-renderer/src/ReactNativeAttributePayload.js b/packages/react-native-renderer/src/ReactNativeAttributePayload.js
index 5804d83b5dc59..fa1187bc339bc 100644
--- a/packages/react-native-renderer/src/ReactNativeAttributePayload.js
+++ b/packages/react-native-renderer/src/ReactNativeAttributePayload.js
@@ -49,7 +49,7 @@ function defaultDiffer(prevProp: mixed, nextProp: mixed): boolean {
 function restoreDeletedValuesInNestedArray(
   updatePayload: Object,
   node: NestedNode,
-  validAttributes: AttributeConfiguration<>,
+  validAttributes: AttributeConfiguration,
 ) {
   if (Array.isArray(node)) {
     let i = node.length;
@@ -107,7 +107,7 @@ function diffNestedArrayProperty(
   updatePayload: null | Object,
   prevArray: Array<NestedNode>,
   nextArray: Array<NestedNode>,
-  validAttributes: AttributeConfiguration<>,
+  validAttributes: AttributeConfiguration,
 ): null | Object {
   const minLength =
     prevArray.length < nextArray.length ? prevArray.length : nextArray.length;
@@ -145,7 +145,7 @@ function diffNestedProperty(
   updatePayload: null | Object,
   prevProp: NestedNode,
   nextProp: NestedNode,
-  validAttributes: AttributeConfiguration<>,
+  validAttributes: AttributeConfiguration,
 ): null | Object {
   if (!updatePayload && prevProp === nextProp) {
     // If no properties have been added, then we can bail out quickly on object
@@ -206,7 +206,7 @@ function diffNestedProperty(
 function addNestedProperty(
   updatePayload: null | Object,
   nextProp: NestedNode,
-  validAttributes: AttributeConfiguration<>,
+  validAttributes: AttributeConfiguration,
 ) {
   if (!nextProp) {
     return updatePayload;
@@ -236,7 +236,7 @@ function addNestedProperty(
 function clearNestedProperty(
   updatePayload: null | Object,
   prevProp: NestedNode,
-  validAttributes: AttributeConfiguration<>,
+  validAttributes: AttributeConfiguration,
 ): null | Object {
   if (!prevProp) {
     return updatePayload;
@@ -268,7 +268,7 @@ function diffProperties(
   updatePayload: null | Object,
   prevProps: Object,
   nextProps: Object,
-  validAttributes: AttributeConfiguration<>,
+  validAttributes: AttributeConfiguration,
 ): null | Object {
   let attributeConfig;
   let nextProp;
@@ -369,13 +369,13 @@ function diffProperties(
         updatePayload,
         prevProp,
         nextProp,
-        ((attributeConfig: any): AttributeConfiguration<>),
+        ((attributeConfig: any): AttributeConfiguration),
       );
       if (removedKeyCount > 0 && updatePayload) {
         restoreDeletedValuesInNestedArray(
           updatePayload,
           nextProp,
-          ((attributeConfig: any): AttributeConfiguration<>),
+          ((attributeConfig: any): AttributeConfiguration),
         );
         removedKeys = null;
       }
@@ -426,7 +426,7 @@ function diffProperties(
       updatePayload = clearNestedProperty(
         updatePayload,
         prevProp,
-        ((attributeConfig: any): AttributeConfiguration<>),
+        ((attributeConfig: any): AttributeConfiguration),
       );
     }
   }
@@ -439,7 +439,7 @@ function diffProperties(
 function addProperties(
   updatePayload: null | Object,
   props: Object,
-  validAttributes: AttributeConfiguration<>,
+  validAttributes: AttributeConfiguration,
 ): null | Object {
   // TODO: Fast path
   return diffProperties(updatePayload, emptyObject, props, validAttributes);
@@ -452,7 +452,7 @@ function addProperties(
 function clearProperties(
   updatePayload: null | Object,
   prevProps: Object,
-  validAttributes: AttributeConfiguration<>,
+  validAttributes: AttributeConfiguration,
 ): null | Object {
   // TODO: Fast path
   return diffProperties(updatePayload, prevProps, emptyObject, validAttributes);
@@ -460,7 +460,7 @@ function clearProperties(
 
 export function create(
   props: Object,
-  validAttributes: AttributeConfiguration<>,
+  validAttributes: AttributeConfiguration,
 ): null | Object {
   return addProperties(
     null, // updatePayload
@@ -472,7 +472,7 @@ export function create(
 export function diff(
   prevProps: Object,
   nextProps: Object,
-  validAttributes: AttributeConfiguration<>,
+  validAttributes: AttributeConfiguration,
 ): null | Object {
   return diffProperties(
     null, // updatePayload
diff --git a/packages/react-native-renderer/src/ReactNativeFiberHostComponent.js b/packages/react-native-renderer/src/ReactNativeFiberHostComponent.js
index a4c90fa3f7be1..8048144431826 100644
--- a/packages/react-native-renderer/src/ReactNativeFiberHostComponent.js
+++ b/packages/react-native-renderer/src/ReactNativeFiberHostComponent.js
@@ -14,7 +14,7 @@ import type {
   MeasureLayoutOnSuccessCallback,
   MeasureOnSuccessCallback,
   NativeMethods,
-  ReactNativeBaseComponentViewConfig,
+  ViewConfig,
 } from './ReactNativeTypes';
 import type {Instance} from './ReactNativeHostConfig';
 
@@ -34,11 +34,11 @@ class ReactNativeFiberHostComponent {
   _children: Array<Instance | number>;
   _nativeTag: number;
   _internalFiberInstanceHandleDEV: Object;
-  viewConfig: ReactNativeBaseComponentViewConfig<>;
+  viewConfig: ViewConfig;
 
   constructor(
     tag: number,
-    viewConfig: ReactNativeBaseComponentViewConfig<>,
+    viewConfig: ViewConfig,
     internalInstanceHandleDEV: Object,
   ) {
     this._nativeTag = tag;
diff --git a/packages/react-native-renderer/src/ReactNativeRenderer.js b/packages/react-native-renderer/src/ReactNativeRenderer.js
index 673482d0175cd..547fdd2de5c52 100644
--- a/packages/react-native-renderer/src/ReactNativeRenderer.js
+++ b/packages/react-native-renderer/src/ReactNativeRenderer.js
@@ -9,6 +9,7 @@
 
 import type {HostComponent} from './ReactNativeTypes';
 import type {ReactNodeList} from 'shared/ReactTypes';
+import type {ElementRef, Element, ElementType} from 'react';
 
 import './ReactNativeInjection';
 
@@ -193,10 +194,10 @@ function sendAccessibilityEvent(handle: any, eventType: string) {
 }
 
 function render(
-  element: React$Element<any>,
-  containerTag: any,
-  callback: ?Function,
-) {
+  element: Element<ElementType>,
+  containerTag: number,
+  callback: ?() => void,
+): ?ElementRef<ElementType> {
   let root = roots.get(containerTag);
 
   if (!root) {
@@ -207,6 +208,7 @@ function render(
   }
   updateContainer(element, root, null, callback);
 
+  // $FlowIssue Flow has hardcoded values for React DOM that don't work with RN
   return getPublicRootInstance(root);
 }
 
diff --git a/packages/react-native-renderer/src/ReactNativeTypes.js b/packages/react-native-renderer/src/ReactNativeTypes.js
index 8b4ab311bff79..462abeac94bee 100644
--- a/packages/react-native-renderer/src/ReactNativeTypes.js
+++ b/packages/react-native-renderer/src/ReactNativeTypes.js
@@ -5,10 +5,10 @@
  * LICENSE file in the root directory of this source tree.
  *
  * @format
- * @flow
+ * @flow strict
  */
 
-import type {ElementRef, AbstractComponent} from 'react';
+import type {ElementRef, ElementType, Element, AbstractComponent} from 'react';
 
 export type MeasureOnSuccessCallback = (
   x: number,
@@ -33,49 +33,66 @@ export type MeasureLayoutOnSuccessCallback = (
   height: number,
 ) => void;
 
-type AttributeType =
+type AttributeType<T, V> =
   | true
   | $ReadOnly<{|
-      diff?: <T>(arg1: T, arg2: T) => boolean,
-      process?: (arg1: any) => any,
+      diff?: (arg1: T, arg2: T) => boolean,
+      process?: (arg1: V) => T,
     |}>;
 
-export type AttributeConfiguration<
-  TProps = string,
-  TStyleProps = string,
-> = $ReadOnly<{
-  [propName: TProps]: AttributeType,
-  style: $ReadOnly<{[propName: TStyleProps]: AttributeType, ...}>,
+// We either force that `diff` and `process` always use mixed,
+// or we allow them to define specific types and use this hack
+type AnyAttributeType = AttributeType<$FlowFixMe, $FlowFixMe>;
+
+export type AttributeConfiguration = $ReadOnly<{
+  [propName: string]: AnyAttributeType,
+  style: $ReadOnly<{
+    [propName: string]: AnyAttributeType,
+    ...,
+  }>,
+  ...
+}>;
+
+type PartialAttributeConfiguration = $ReadOnly<{
+  [propName: string]: AnyAttributeType,
+  style?: $ReadOnly<{
+    [propName: string]: AnyAttributeType,
+    ...,
+  }>,
   ...
 }>;
 
-export type ReactNativeBaseComponentViewConfig<
-  TProps = string,
-  TStyleProps = string,
-> = $ReadOnly<{|
-  baseModuleName?: string,
+export type ViewConfig = $ReadOnly<{
+  Commands?: $ReadOnly<{[commandName: string]: number, ...}>,
+  Constants?: $ReadOnly<{[name: string]: mixed, ...}>,
+  Manager?: string,
+  NativeProps?: $ReadOnly<{[propName: string]: string, ...}>,
+  baseModuleName?: ?string,
   bubblingEventTypes?: $ReadOnly<{
-    [eventName: string]: $ReadOnly<{|
-      phasedRegistrationNames: $ReadOnly<{|
+    [eventName: string]: $ReadOnly<{
+      phasedRegistrationNames: $ReadOnly<{
         captured: string,
         bubbled: string,
-      |}>,
-    |}>,
+      }>,
+    }>,
     ...,
   }>,
-  Commands?: $ReadOnly<{[commandName: string]: number, ...}>,
   directEventTypes?: $ReadOnly<{
-    [eventName: string]: $ReadOnly<{|
+    [eventName: string]: $ReadOnly<{
       registrationName: string,
-    |}>,
+    }>,
     ...,
   }>,
-  NativeProps?: $ReadOnly<{[propName: string]: string, ...}>,
   uiViewClassName: string,
-  validAttributes: AttributeConfiguration<TProps, TStyleProps>,
-|}>;
+  validAttributes: AttributeConfiguration,
+}>;
 
-export type ViewConfigGetter = () => ReactNativeBaseComponentViewConfig<>;
+export type PartialViewConfig = $ReadOnly<{
+  bubblingEventTypes?: $PropertyType<ViewConfig, 'bubblingEventTypes'>,
+  directEventTypes?: $PropertyType<ViewConfig, 'directEventTypes'>,
+  uiViewClassName: string,
+  validAttributes?: PartialAttributeConfiguration,
+}>;
 
 export type NativeMethods = {
   blur(): void,
@@ -87,7 +104,7 @@ export type NativeMethods = {
     onSuccess: MeasureLayoutOnSuccessCallback,
     onFail?: () => void,
   ): void,
-  setNativeProps(nativeProps: Object): void,
+  setNativeProps(nativeProps: {...}): void,
   ...
 };
 
@@ -111,9 +128,11 @@ type InspectorDataSource = $ReadOnly<{|
 |}>;
 
 type InspectorDataGetter = (
-  (componentOrHandle: any) => ?number,
+  <TElementType: ElementType>(
+    componentOrHandle: ElementRef<TElementType> | number,
+  ) => ?number,
 ) => $ReadOnly<{|
-  measure: Function,
+  measure: (callback: MeasureOnSuccessCallback) => void,
   props: InspectorDataProps,
   source: InspectorDataSource,
 |}>;
@@ -145,62 +164,65 @@ export type TouchedViewDataAtPoint = $ReadOnly<{|
  * Provide minimal Flow typing for the high-level RN API and call it a day.
  */
 export type ReactNativeType = {
-  findHostInstance_DEPRECATED(
-    componentOrHandle: any,
+  findHostInstance_DEPRECATED<TElementType: ElementType>(
+    componentOrHandle: ?(ElementRef<TElementType> | number),
   ): ?ElementRef<HostComponent<mixed>>,
-  findNodeHandle(componentOrHandle: any): ?number,
+  findNodeHandle<TElementType: ElementType>(
+    componentOrHandle: ?(ElementRef<TElementType> | number),
+  ): ?number,
   dispatchCommand(
     handle: ElementRef<HostComponent<mixed>>,
     command: string,
-    args: Array<any>,
+    args: Array<mixed>,
   ): void,
   sendAccessibilityEvent(
     handle: ElementRef<HostComponent<mixed>>,
     eventType: string,
   ): void,
   render(
-    element: React$Element<any>,
-    containerTag: any,
-    callback: ?Function,
-  ): any,
-  unmountComponentAtNode(containerTag: number): any,
-  unmountComponentAtNodeAndRemoveContainer(containerTag: number): any,
-  // TODO (bvaughn) Add types
-  unstable_batchedUpdates: any,
+    element: Element<ElementType>,
+    containerTag: number,
+    callback: ?() => void,
+  ): ?ElementRef<ElementType>,
+  unmountComponentAtNode(containerTag: number): void,
+  unmountComponentAtNodeAndRemoveContainer(containerTag: number): void,
+  unstable_batchedUpdates: <T>(fn: (T) => void, bookkeeping: T) => void,
   __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: SecretInternalsType,
   ...
 };
 
 export type ReactFabricType = {
-  findHostInstance_DEPRECATED(
-    componentOrHandle: any,
+  findHostInstance_DEPRECATED<TElementType: ElementType>(
+    componentOrHandle: ?(ElementRef<TElementType> | number),
   ): ?ElementRef<HostComponent<mixed>>,
-  findNodeHandle(componentOrHandle: any): ?number,
+  findNodeHandle<TElementType: ElementType>(
+    componentOrHandle: ?(ElementRef<TElementType> | number),
+  ): ?number,
   dispatchCommand(
     handle: ElementRef<HostComponent<mixed>>,
     command: string,
-    args: Array<any>,
+    args: Array<mixed>,
   ): void,
   sendAccessibilityEvent(
     handle: ElementRef<HostComponent<mixed>>,
     eventType: string,
   ): void,
   render(
-    element: React$Element<any>,
-    containerTag: any,
-    callback: ?Function,
-  ): any,
-  unmountComponentAtNode(containerTag: number): any,
+    element: Element<ElementType>,
+    containerTag: number,
+    callback: ?() => void,
+  ): ?ElementRef<ElementType>,
+  unmountComponentAtNode(containerTag: number): void,
   ...
 };
 
 export type ReactNativeEventTarget = {
-  node: Object,
+  node: {...},
   canonical: {
     _nativeTag: number,
-    viewConfig: ReactNativeBaseComponentViewConfig<>,
-    currentProps: Object,
-    _internalInstanceHandle: Object,
+    viewConfig: ViewConfig,
+    currentProps: {...},
+    _internalInstanceHandle: {...},
     ...
   },
   ...
diff --git a/packages/react-reconciler/src/SchedulerWithReactIntegration.new.js b/packages/react-reconciler/src/SchedulerWithReactIntegration.new.js
index 36dbc704fc7cd..1c13e3a8fda90 100644
--- a/packages/react-reconciler/src/SchedulerWithReactIntegration.new.js
+++ b/packages/react-reconciler/src/SchedulerWithReactIntegration.new.js
@@ -16,6 +16,7 @@ import {__interactionsRef} from 'scheduler/tracing';
 import {
   enableSchedulerTracing,
   decoupleUpdatePriorityFromScheduler,
+  enableSyncMicroTasks,
 } from 'shared/ReactFeatureFlags';
 import invariant from 'shared/invariant';
 import {
@@ -23,6 +24,7 @@ import {
   getCurrentUpdateLanePriority,
   setCurrentUpdateLanePriority,
 } from './ReactFiberLane.new';
+import {scheduleMicrotask, supportsMicrotasks} from './ReactFiberHostConfig';
 
 const {
   unstable_runWithPriority: Scheduler_runWithPriority,
@@ -144,13 +146,19 @@ export function scheduleSyncCallback(callback: SchedulerCallback) {
   // the next tick, or earlier if something calls `flushSyncCallbackQueue`.
   if (syncQueue === null) {
     syncQueue = [callback];
-    // Flush the queue in the next tick, at the earliest.
+
     // TODO: Figure out how to remove this It's only here as a last resort if we
     // forget to explicitly flush.
-    immediateQueueCallbackNode = Scheduler_scheduleCallback(
-      Scheduler_ImmediatePriority,
-      flushSyncCallbackQueueImpl,
-    );
+    if (enableSyncMicroTasks && supportsMicrotasks) {
+      // Flush the queue in a microtask.
+      scheduleMicrotask(flushSyncCallbackQueueImpl);
+    } else {
+      // Flush the queue in the next tick.
+      immediateQueueCallbackNode = Scheduler_scheduleCallback(
+        Scheduler_ImmediatePriority,
+        flushSyncCallbackQueueImpl,
+      );
+    }
   } else {
     // Push onto existing queue. Don't need to schedule a callback because
     // we already scheduled one when we created the queue.
diff --git a/packages/react-reconciler/src/SchedulerWithReactIntegration.old.js b/packages/react-reconciler/src/SchedulerWithReactIntegration.old.js
index 1d309df4020f1..5c4451499ca5b 100644
--- a/packages/react-reconciler/src/SchedulerWithReactIntegration.old.js
+++ b/packages/react-reconciler/src/SchedulerWithReactIntegration.old.js
@@ -16,6 +16,7 @@ import {__interactionsRef} from 'scheduler/tracing';
 import {
   enableSchedulerTracing,
   decoupleUpdatePriorityFromScheduler,
+  enableSyncMicroTasks,
 } from 'shared/ReactFeatureFlags';
 import invariant from 'shared/invariant';
 import {
@@ -23,6 +24,7 @@ import {
   getCurrentUpdateLanePriority,
   setCurrentUpdateLanePriority,
 } from './ReactFiberLane.old';
+import {scheduleMicrotask, supportsMicrotasks} from './ReactFiberHostConfig';
 
 const {
   unstable_runWithPriority: Scheduler_runWithPriority,
@@ -144,13 +146,19 @@ export function scheduleSyncCallback(callback: SchedulerCallback) {
   // the next tick, or earlier if something calls `flushSyncCallbackQueue`.
   if (syncQueue === null) {
     syncQueue = [callback];
-    // Flush the queue in the next tick, at the earliest.
+
     // TODO: Figure out how to remove this It's only here as a last resort if we
     // forget to explicitly flush.
-    immediateQueueCallbackNode = Scheduler_scheduleCallback(
-      Scheduler_ImmediatePriority,
-      flushSyncCallbackQueueImpl,
-    );
+    if (enableSyncMicroTasks && supportsMicrotasks) {
+      // Flush the queue in a microtask.
+      scheduleMicrotask(flushSyncCallbackQueueImpl);
+    } else {
+      // Flush the queue in the next tick.
+      immediateQueueCallbackNode = Scheduler_scheduleCallback(
+        Scheduler_ImmediatePriority,
+        flushSyncCallbackQueueImpl,
+      );
+    }
   } else {
     // Push onto existing queue. Don't need to schedule a callback because
     // we already scheduled one when we created the queue.
diff --git a/packages/react-reconciler/src/__tests__/ReactExpiration-test.js b/packages/react-reconciler/src/__tests__/ReactExpiration-test.js
index 017c712bd814b..48ff50880f943 100644
--- a/packages/react-reconciler/src/__tests__/ReactExpiration-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactExpiration-test.js
@@ -598,12 +598,13 @@ describe('ReactExpiration', () => {
       // second one.
       Scheduler.unstable_advanceTime(1000);
       // Attempt to interrupt with a high pri update.
-      updateHighPri();
+      await ReactNoop.act(async () => {
+        updateHighPri();
+      });
 
-      // The first update expired, so first will finish it without
-      // interrupting. But not the second update, which hasn't expired yet.
-      expect(Scheduler).toFlushExpired(['Sibling']);
-      expect(Scheduler).toFlushAndYield([
+      expect(Scheduler).toHaveYielded([
+        // The first update expired
+        'Sibling',
         // Then render the high pri update
         'High pri: 1',
         'Normal pri: 1',
diff --git a/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.js b/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.js
index bf5503feb4902..4c7d308a9c909 100644
--- a/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.js
@@ -1792,7 +1792,7 @@ describe('ReactHooksWithNoopRenderer', () => {
     it(
       'in legacy mode, useEffect is deferred and updates finish synchronously ' +
         '(in a single batch)',
-      () => {
+      async () => {
         function Counter(props) {
           const [count, updateCount] = useState('(empty)');
           useEffect(() => {
@@ -1807,10 +1807,12 @@ describe('ReactHooksWithNoopRenderer', () => {
           }, [props.count]);
           return <Text text={'Count: ' + count} />;
         }
-        act(() => {
+        await act(async () => {
           ReactNoop.renderLegacySyncRoot(<Counter count={0} />);
+
           // Even in legacy mode, effects are deferred until after paint
-          expect(Scheduler).toFlushAndYieldThrough(['Count: (empty)']);
+          ReactNoop.flushSync();
+          expect(Scheduler).toHaveYielded(['Count: (empty)']);
           expect(ReactNoop.getChildren()).toEqual([span('Count: (empty)')]);
         });
 
diff --git a/packages/react-reconciler/src/__tests__/ReactIncrementalErrorHandling-test.internal.js b/packages/react-reconciler/src/__tests__/ReactIncrementalErrorHandling-test.internal.js
index f568191681b67..613fe89d63279 100644
--- a/packages/react-reconciler/src/__tests__/ReactIncrementalErrorHandling-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactIncrementalErrorHandling-test.internal.js
@@ -1400,6 +1400,10 @@ describe('ReactIncrementalErrorHandling', () => {
       'BrokenRenderAndUnmount componentWillUnmount',
     ]);
     expect(ReactNoop.getChildren()).toEqual([]);
+
+    expect(() => {
+      ReactNoop.flushSync();
+    }).toThrow('One does not simply unmount me.');
   });
 
   it('does not interrupt unmounting if detaching a ref throws', () => {
diff --git a/packages/react-reconciler/src/__tests__/ReactOffscreen-test.js b/packages/react-reconciler/src/__tests__/ReactOffscreen-test.js
index e830fa2b24ddf..b3f14e6ad1efc 100644
--- a/packages/react-reconciler/src/__tests__/ReactOffscreen-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactOffscreen-test.js
@@ -98,8 +98,11 @@ describe('ReactOffscreen', () => {
           <Text text="Outside" />
         </>,
       );
+
+      ReactNoop.flushSync();
+
       // Should not defer the hidden tree
-      expect(Scheduler).toFlushUntilNextPaint(['A', 'Outside']);
+      expect(Scheduler).toHaveYielded(['A', 'Outside']);
     });
     expect(root).toMatchRenderedOutput(
       <>
diff --git a/packages/react-reconciler/src/__tests__/ReactSchedulerIntegration-test.js b/packages/react-reconciler/src/__tests__/ReactSchedulerIntegration-test.js
index 4292333293e82..ab9995d01cb5f 100644
--- a/packages/react-reconciler/src/__tests__/ReactSchedulerIntegration-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactSchedulerIntegration-test.js
@@ -569,9 +569,11 @@ describe(
           ReactNoop.render(<App />);
         });
 
+        ReactNoop.flushSync();
+
         // Because the render expired, React should finish the tree without
         // consulting `shouldYield` again
-        expect(Scheduler).toFlushExpired(['B', 'C']);
+        expect(Scheduler).toHaveYielded(['B', 'C']);
       });
     });
   },
diff --git a/packages/react-reconciler/src/__tests__/ReactSuspenseList-test.js b/packages/react-reconciler/src/__tests__/ReactSuspenseList-test.js
index de74c18e5ca6a..2cdc2621fbe9a 100644
--- a/packages/react-reconciler/src/__tests__/ReactSuspenseList-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactSuspenseList-test.js
@@ -292,9 +292,11 @@ describe('ReactSuspenseList', () => {
       </>,
     );
 
-    await C.resolve();
+    await ReactNoop.act(async () => {
+      C.resolve();
+    });
 
-    expect(Scheduler).toFlushAndYield(['C']);
+    expect(Scheduler).toHaveYielded(['C']);
 
     expect(ReactNoop).toMatchRenderedOutput(
       <>
@@ -304,9 +306,11 @@ describe('ReactSuspenseList', () => {
       </>,
     );
 
-    await B.resolve();
+    await ReactNoop.act(async () => {
+      B.resolve();
+    });
 
-    expect(Scheduler).toFlushAndYield(['B']);
+    expect(Scheduler).toHaveYielded(['B']);
 
     expect(ReactNoop).toMatchRenderedOutput(
       <>
diff --git a/packages/react-reconciler/src/__tests__/ReactSuspensePlaceholder-test.internal.js b/packages/react-reconciler/src/__tests__/ReactSuspensePlaceholder-test.internal.js
index d40f56380b7e8..b6e34104ad61f 100644
--- a/packages/react-reconciler/src/__tests__/ReactSuspensePlaceholder-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactSuspensePlaceholder-test.internal.js
@@ -310,7 +310,7 @@ describe('ReactSuspensePlaceholder', () => {
     });
 
     describe('when suspending during mount', () => {
-      it('properly accounts for base durations when a suspended times out in a legacy tree', () => {
+      it('properly accounts for base durations when a suspended times out in a legacy tree', async () => {
         ReactNoop.renderLegacySyncRoot(<App shouldSuspend={true} />);
         expect(Scheduler).toHaveYielded([
           'App',
@@ -331,7 +331,10 @@ describe('ReactSuspensePlaceholder', () => {
         jest.advanceTimersByTime(1000);
 
         expect(Scheduler).toHaveYielded(['Promise resolved [Loaded]']);
-        expect(Scheduler).toFlushExpired(['Loaded']);
+
+        ReactNoop.flushSync();
+
+        expect(Scheduler).toHaveYielded(['Loaded']);
         expect(ReactNoop).toMatchRenderedOutput('LoadedText');
         expect(onRender).toHaveBeenCalledTimes(2);
 
@@ -378,7 +381,7 @@ describe('ReactSuspensePlaceholder', () => {
     });
 
     describe('when suspending during update', () => {
-      it('properly accounts for base durations when a suspended times out in a legacy tree', () => {
+      it('properly accounts for base durations when a suspended times out in a legacy tree', async () => {
         ReactNoop.renderLegacySyncRoot(
           <App shouldSuspend={false} textRenderDuration={5} />,
         );
@@ -427,7 +430,10 @@ describe('ReactSuspensePlaceholder', () => {
         jest.advanceTimersByTime(1000);
 
         expect(Scheduler).toHaveYielded(['Promise resolved [Loaded]']);
-        expect(Scheduler).toFlushExpired(['Loaded']);
+
+        ReactNoop.flushSync();
+
+        expect(Scheduler).toHaveYielded(['Loaded']);
         expect(ReactNoop).toMatchRenderedOutput('LoadedNew');
         expect(onRender).toHaveBeenCalledTimes(4);
 
diff --git a/packages/react-reconciler/src/__tests__/ReactSuspenseWithNoopRenderer-test.js b/packages/react-reconciler/src/__tests__/ReactSuspenseWithNoopRenderer-test.js
index 20971e9f58e91..8e8a512dacf41 100644
--- a/packages/react-reconciler/src/__tests__/ReactSuspenseWithNoopRenderer-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactSuspenseWithNoopRenderer-test.js
@@ -1088,9 +1088,11 @@ describe('ReactSuspenseWithNoopRenderer', () => {
       expect(Scheduler).toHaveYielded(['Suspend! [Result]', 'Loading...']);
       expect(ReactNoop.getChildren()).toEqual([span('Loading...')]);
 
-      await resolveText('Result');
+      await ReactNoop.act(async () => {
+        resolveText('Result');
+      });
 
-      expect(Scheduler).toFlushExpired(['Result']);
+      expect(Scheduler).toHaveYielded(['Result']);
       expect(ReactNoop.getChildren()).toEqual([span('Result')]);
     });
 
@@ -1156,8 +1158,10 @@ describe('ReactSuspenseWithNoopRenderer', () => {
         </>,
       );
 
-      await resolveText('Step: 2');
-      expect(Scheduler).toFlushExpired(['Step: 2']);
+      await ReactNoop.act(async () => {
+        resolveText('Step: 2');
+      });
+      expect(Scheduler).toHaveYielded(['Step: 2']);
       expect(ReactNoop).toMatchRenderedOutput(
         <>
           <span prop="Step: 2" />
@@ -1227,9 +1231,11 @@ describe('ReactSuspenseWithNoopRenderer', () => {
         </>,
       );
 
-      await resolveText('B');
+      await ReactNoop.act(async () => {
+        resolveText('B');
+      });
 
-      expect(Scheduler).toFlushExpired(['B']);
+      expect(Scheduler).toHaveYielded(['B']);
       expect(ReactNoop).toMatchRenderedOutput(
         <>
           <span prop="A" />
@@ -1271,9 +1277,11 @@ describe('ReactSuspenseWithNoopRenderer', () => {
       ]);
       expect(ReactNoop.getChildren()).toEqual([span('Loading...')]);
 
-      await resolveText('Hi');
+      await ReactNoop.act(async () => {
+        resolveText('Hi');
+      });
 
-      expect(Scheduler).toFlushExpired([
+      expect(Scheduler).toHaveYielded([
         'constructor',
         'Hi',
         'componentDidMount',
@@ -1316,8 +1324,10 @@ describe('ReactSuspenseWithNoopRenderer', () => {
         'Loading...',
       ]);
       expect(ReactNoop.getChildren()).toEqual([span('Loading...')]);
-      await resolveText('Hi');
-      expect(Scheduler).toFlushExpired(['Hi']);
+      await ReactNoop.act(async () => {
+        resolveText('Hi');
+      });
+      expect(Scheduler).toHaveYielded(['Hi']);
       expect(ReactNoop.getChildren()).toEqual([span('Hi')]);
     });
 
@@ -1360,8 +1370,10 @@ describe('ReactSuspenseWithNoopRenderer', () => {
           </>,
         ]);
 
-        await resolveText('Hi');
-        expect(Scheduler).toFlushExpired(['Hi']);
+        await ReactNoop.act(async () => {
+          resolveText('Hi');
+        });
+        expect(Scheduler).toHaveYielded(['Hi']);
       });
     } else {
       // @gate enableCache
@@ -1401,9 +1413,11 @@ describe('ReactSuspenseWithNoopRenderer', () => {
           'Child is hidden: true',
         ]);
 
-        await resolveText('Hi');
+        await ReactNoop.act(async () => {
+          resolveText('Hi');
+        });
 
-        expect(Scheduler).toFlushExpired(['Hi']);
+        expect(Scheduler).toHaveYielded(['Hi']);
       });
     }
 
@@ -1647,9 +1661,11 @@ describe('ReactSuspenseWithNoopRenderer', () => {
       </>,
     );
 
-    await resolveText('B');
+    await ReactNoop.act(async () => {
+      resolveText('B');
+    });
 
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'B',
       'Destroy Layout Effect [Loading...]',
       'Layout Effect [B]',
@@ -1681,9 +1697,11 @@ describe('ReactSuspenseWithNoopRenderer', () => {
       'Effect [Loading...]',
     ]);
 
-    await resolveText('B2');
+    await ReactNoop.act(async () => {
+      resolveText('B2');
+    });
 
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'B2',
       'Destroy Layout Effect [Loading...]',
       'Destroy Layout Effect [B]',
diff --git a/packages/react-reconciler/src/__tests__/useMutableSource-test.internal.js b/packages/react-reconciler/src/__tests__/useMutableSource-test.internal.js
index db4214bfd0656..cf1db2891e11d 100644
--- a/packages/react-reconciler/src/__tests__/useMutableSource-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/useMutableSource-test.internal.js
@@ -1794,7 +1794,7 @@ describe('useMutableSource', () => {
       });
 
       // @gate experimental
-      it('should not misidentify mutations after render as side effects', () => {
+      it('should not misidentify mutations after render as side effects', async () => {
         const source = createSource('initial');
         const mutableSource = createMutableSource(
           source,
@@ -1811,15 +1811,16 @@ describe('useMutableSource', () => {
           return null;
         }
 
-        act(() => {
+        await act(async () => {
           ReactNoop.renderLegacySyncRoot(
             <React.StrictMode>
               <MutateDuringRead />
             </React.StrictMode>,
           );
-          expect(Scheduler).toFlushAndYieldThrough([
-            'MutateDuringRead:initial',
-          ]);
+        });
+        expect(Scheduler).toHaveYielded(['MutateDuringRead:initial']);
+
+        await act(async () => {
           source.value = 'updated';
         });
         expect(Scheduler).toHaveYielded(['MutateDuringRead:updated']);
diff --git a/packages/shared/ReactFeatureFlags.js b/packages/shared/ReactFeatureFlags.js
index 953eb7cd56615..201546e822b20 100644
--- a/packages/shared/ReactFeatureFlags.js
+++ b/packages/shared/ReactFeatureFlags.js
@@ -155,4 +155,6 @@ export const disableSchedulerTimeoutInWorkLoop = false;
 
 export const enableDiscreteEventMicroTasks = false;
 
+export const enableSyncMicroTasks = false;
+
 export const enableNativeEventPriorityInference = false;
diff --git a/packages/shared/forks/ReactFeatureFlags.native-fb.js b/packages/shared/forks/ReactFeatureFlags.native-fb.js
index 2f4406b2ceafa..e0bff4b32f634 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-fb.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-fb.js
@@ -59,6 +59,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableDiscreteEventMicroTasks = false;
+export const enableSyncMicroTasks = false;
 export const enableNativeEventPriorityInference = false;
 
 // Flow magic to verify the exports of this file match the original version.
diff --git a/packages/shared/forks/ReactFeatureFlags.native-oss.js b/packages/shared/forks/ReactFeatureFlags.native-oss.js
index 35e1067646d94..a40b6e293c241 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-oss.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-oss.js
@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableDiscreteEventMicroTasks = false;
+export const enableSyncMicroTasks = false;
 export const enableNativeEventPriorityInference = false;
 
 // Flow magic to verify the exports of this file match the original version.
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
index 67ecdb72983c7..97a26ddd86819 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableDiscreteEventMicroTasks = false;
+export const enableSyncMicroTasks = false;
 export const enableNativeEventPriorityInference = false;
 
 // Flow magic to verify the exports of this file match the original version.
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.native.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.native.js
index 9ed0be59689e7..1417670d7915c 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.native.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.native.js
@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableDiscreteEventMicroTasks = false;
+export const enableSyncMicroTasks = false;
 export const enableNativeEventPriorityInference = false;
 
 // Flow magic to verify the exports of this file match the original version.
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
index 2e7513a8ddf15..17848a59376c6 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableDiscreteEventMicroTasks = false;
+export const enableSyncMicroTasks = false;
 export const enableNativeEventPriorityInference = false;
 
 // Flow magic to verify the exports of this file match the original version.
diff --git a/packages/shared/forks/ReactFeatureFlags.testing.js b/packages/shared/forks/ReactFeatureFlags.testing.js
index 5a362e8a31419..ce73c8a870cf0 100644
--- a/packages/shared/forks/ReactFeatureFlags.testing.js
+++ b/packages/shared/forks/ReactFeatureFlags.testing.js
@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableDiscreteEventMicroTasks = false;
+export const enableSyncMicroTasks = false;
 export const enableNativeEventPriorityInference = false;
 
 // Flow magic to verify the exports of this file match the original version.
diff --git a/packages/shared/forks/ReactFeatureFlags.testing.www.js b/packages/shared/forks/ReactFeatureFlags.testing.www.js
index 54942f0c7cd2d..02dba8551fb1a 100644
--- a/packages/shared/forks/ReactFeatureFlags.testing.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.testing.www.js
@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableDiscreteEventMicroTasks = false;
+export const enableSyncMicroTasks = false;
 export const enableNativeEventPriorityInference = false;
 
 // Flow magic to verify the exports of this file match the original version.
diff --git a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
index 1d9f89699f317..b75b80dbde650 100644
--- a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
+++ b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
@@ -57,4 +57,5 @@ export const enableUseRefAccessWarning = __VARIANT__;
 export const enableProfilerNestedUpdateScheduledHook = __VARIANT__;
 export const disableSchedulerTimeoutInWorkLoop = __VARIANT__;
 export const enableDiscreteEventMicroTasks = __VARIANT__;
+export const enableSyncMicroTasks = __VARIANT__;
 export const enableNativeEventPriorityInference = __VARIANT__;
diff --git a/packages/shared/forks/ReactFeatureFlags.www.js b/packages/shared/forks/ReactFeatureFlags.www.js
index 5e5cd2105b383..0612eacf80d07 100644
--- a/packages/shared/forks/ReactFeatureFlags.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.www.js
@@ -33,6 +33,7 @@ export const {
   disableNativeComponentFrames,
   disableSchedulerTimeoutInWorkLoop,
   enableDiscreteEventMicroTasks,
+  enableSyncMicroTasks,
   enableNativeEventPriorityInference,
 } = dynamicFeatureFlags;
 
diff --git a/scripts/rollup/shims/react-native/ReactNativeViewConfigRegistry.js b/scripts/rollup/shims/react-native/ReactNativeViewConfigRegistry.js
index 2c09ddf3caab1..5f219fea65208 100644
--- a/scripts/rollup/shims/react-native/ReactNativeViewConfigRegistry.js
+++ b/scripts/rollup/shims/react-native/ReactNativeViewConfigRegistry.js
@@ -12,12 +12,8 @@
 
 'use strict';
 
-import type {
-  ReactNativeBaseComponentViewConfig,
-  ViewConfigGetter,
-} from './ReactNativeTypes';
-
-const invariant = require('invariant');
+import {type ViewConfig} from './ReactNativeTypes';
+import invariant from 'invariant';
 
 // Event configs
 const customBubblingEventTypes: {
@@ -42,9 +38,7 @@ exports.customDirectEventTypes = customDirectEventTypes;
 const viewConfigCallbacks = new Map();
 const viewConfigs = new Map();
 
-function processEventTypes(
-  viewConfig: ReactNativeBaseComponentViewConfig<>,
-): void {
+function processEventTypes(viewConfig: ViewConfig): void {
   const {bubblingEventTypes, directEventTypes} = viewConfig;
 
   if (__DEV__) {
@@ -82,7 +76,7 @@ function processEventTypes(
  * A callback is provided to load the view config from UIManager.
  * The callback is deferred until the view is actually rendered.
  */
-exports.register = function(name: string, callback: ViewConfigGetter): string {
+exports.register = function(name: string, callback: () => ViewConfig): string {
   invariant(
     !viewConfigCallbacks.has(name),
     'Tried to register two views with the same name %s',
@@ -103,7 +97,7 @@ exports.register = function(name: string, callback: ViewConfigGetter): string {
  * If this is the first time the view has been used,
  * This configuration will be lazy-loaded from UIManager.
  */
-exports.get = function(name: string): ReactNativeBaseComponentViewConfig<> {
+exports.get = function(name: string): ViewConfig {
   let viewConfig;
   if (!viewConfigs.has(name)) {
     const callback = viewConfigCallbacks.get(name);
diff --git a/scripts/rollup/shims/react-native/createReactNativeComponentClass.js b/scripts/rollup/shims/react-native/createReactNativeComponentClass.js
index 86a758d918b48..f8f4c9284e4c3 100644
--- a/scripts/rollup/shims/react-native/createReactNativeComponentClass.js
+++ b/scripts/rollup/shims/react-native/createReactNativeComponentClass.js
@@ -11,8 +11,7 @@
 'use strict';
 
 import {ReactNativeViewConfigRegistry} from 'react-native/Libraries/ReactPrivate/ReactNativePrivateInterface';
-
-import type {ViewConfigGetter} from './ReactNativeTypes';
+import {type ViewConfig} from './ReactNativeTypes';
 
 const {register} = ReactNativeViewConfigRegistry;
 
@@ -26,7 +25,7 @@ const {register} = ReactNativeViewConfigRegistry;
  */
 const createReactNativeComponentClass = function(
   name: string,
-  callback: ViewConfigGetter,
+  callback: () => ViewConfig,
 ): string {
   return register(name, callback);
 };