RFC Implementation: Speculative work

packages/react-reconciler/src/ReactFiber.js

@@ -169,6 +169,9 @@ export type Fiber = {|
   sibling: Fiber | null,
   index: number,
 
+  // identifier of last rendered children
+  residue: any,
+
   // The ref last used to attach this node.
   // I'll avoid adding an owner field for prod and model that as functions.
   ref:
@@ -277,6 +280,8 @@ function FiberNode(
   this.sibling = null;
   this.index = 0;
 
+  this.residue = null;
+
   this.ref = null;
 
   this.pendingProps = pendingProps;
@@ -367,6 +372,8 @@ const createFiber = function(
   return new FiberNode(tag, pendingProps, key, mode);
 };
 
+export const voidFiber = createFiber(0, null, null, 0);
+
 function shouldConstruct(Component: Function) {
   const prototype = Component.prototype;
   return !!(prototype && prototype.isReactComponent);
@@ -465,6 +472,8 @@ export function createWorkInProgress(
   workInProgress.memoizedState = current.memoizedState;
   workInProgress.updateQueue = current.updateQueue;
 
+  workInProgress.residue = current.residue;
+
   // Clone the dependencies object. This is mutated during the render phase, so
   // it cannot be shared with the current fiber.
   const currentDependencies = current.dependencies;