diff --git a/packages/@lwc/engine-core/src/index.ts b/packages/@lwc/engine-core/src/index.ts
index c7d536e4a6..4f483cfa5d 100644
--- a/packages/@lwc/engine-core/src/index.ts
+++ b/packages/@lwc/engine-core/src/index.ts
@@ -5,4 +5,7 @@
  * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
  */
 
+// Tests
+import './testFeatureFlag';
+
 export * from './framework/main';
diff --git a/packages/@lwc/engine-core/src/testFeatureFlag.ts b/packages/@lwc/engine-core/src/testFeatureFlag.ts
new file mode 100644
index 0000000000..d7b4b6c551
--- /dev/null
+++ b/packages/@lwc/engine-core/src/testFeatureFlag.ts
@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2018, salesforce.com, inc.
+ * All rights reserved.
+ * SPDX-License-Identifier: MIT
+ * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
+ */
+
+import features from '@lwc/features';
+
+if (process.env.NODE_ENV !== 'production' && typeof window !== 'undefined') {
+    window.addEventListener('test-dummy-flag', () => {
+        let hasFlag = false;
+        if (features.DUMMY_TEST_FLAG) {
+            hasFlag = true;
+        }
+
+        window.dispatchEvent(
+            new CustomEvent('has-dummy-flag', {
+                detail: {
+                    package: '@lwc/engine-core',
+                    hasFlag,
+                },
+            })
+        );
+    });
+}
diff --git a/packages/@lwc/engine-dom/scripts/rollup.config.js b/packages/@lwc/engine-dom/scripts/rollup.config.js
index 25260078a2..728d5fe988 100644
--- a/packages/@lwc/engine-dom/scripts/rollup.config.js
+++ b/packages/@lwc/engine-dom/scripts/rollup.config.js
@@ -11,6 +11,7 @@ const path = require('path');
 const { nodeResolve } = require('@rollup/plugin-node-resolve');
 const typescript = require('../../../../scripts/rollup/typescript');
 const writeDistAndTypes = require('../../../../scripts/rollup/writeDistAndTypes');
+const lwcFeatures = require('../../../../scripts/rollup/lwcFeatures');
 const { version } = require('../package.json');
 
 const banner = `/* proxy-compat-disable */`;
@@ -35,6 +36,7 @@ module.exports = {
         }),
         typescript(),
         writeDistAndTypes(),
+        lwcFeatures(),
     ],
 
     onwarn({ code, message }) {
diff --git a/packages/@lwc/engine-dom/src/index.ts b/packages/@lwc/engine-dom/src/index.ts
index ca90813c0b..f06f545b6e 100644
--- a/packages/@lwc/engine-dom/src/index.ts
+++ b/packages/@lwc/engine-dom/src/index.ts
@@ -11,6 +11,9 @@ import './polyfills/aria-properties/main';
 // Renderer initialization -------------------------------------------------------------------------
 import './initializeRenderer';
 
+// Tests -------------------------------------------------------------------------------------------
+import './testFeatureFlag.ts';
+
 // Engine-core public APIs -------------------------------------------------------------------------
 export {
     createContextProvider,
diff --git a/packages/@lwc/engine-dom/src/testFeatureFlag.ts b/packages/@lwc/engine-dom/src/testFeatureFlag.ts
new file mode 100644
index 0000000000..6489948e5e
--- /dev/null
+++ b/packages/@lwc/engine-dom/src/testFeatureFlag.ts
@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2018, salesforce.com, inc.
+ * All rights reserved.
+ * SPDX-License-Identifier: MIT
+ * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
+ */
+
+import features from '@lwc/features';
+
+if (process.env.NODE_ENV !== 'production' && typeof window !== 'undefined') {
+    window.addEventListener('test-dummy-flag', () => {
+        let hasFlag = false;
+        if (features.DUMMY_TEST_FLAG) {
+            hasFlag = true;
+        }
+
+        window.dispatchEvent(
+            new CustomEvent('has-dummy-flag', {
+                detail: {
+                    package: '@lwc/engine-dom',
+                    hasFlag,
+                },
+            })
+        );
+    });
+}
diff --git a/packages/@lwc/engine-server/scripts/rollup.config.js b/packages/@lwc/engine-server/scripts/rollup.config.js
index fb155beb1c..60ee1f1f91 100644
--- a/packages/@lwc/engine-server/scripts/rollup.config.js
+++ b/packages/@lwc/engine-server/scripts/rollup.config.js
@@ -9,6 +9,7 @@ const path = require('path');
 const { nodeResolve } = require('@rollup/plugin-node-resolve');
 const typescript = require('../../../../scripts/rollup/typescript');
 const writeDistAndTypes = require('../../../../scripts/rollup/writeDistAndTypes');
+const lwcFeatures = require('../../../../scripts/rollup/lwcFeatures');
 const { version } = require('../package.json');
 
 const banner = `/* proxy-compat-disable */`;
@@ -33,6 +34,7 @@ module.exports = {
         }),
         typescript(),
         writeDistAndTypes(),
+        lwcFeatures(),
     ],
 
     onwarn({ code, message }) {
diff --git a/packages/@lwc/features/src/flags.ts b/packages/@lwc/features/src/flags.ts
index e31c03932f..04ff3d6e16 100644
--- a/packages/@lwc/features/src/flags.ts
+++ b/packages/@lwc/features/src/flags.ts
@@ -8,6 +8,7 @@ import { create, keys, defineProperty, isUndefined, isBoolean, globalThis } from
 import { FeatureFlagMap, FeatureFlagName, FeatureFlagValue } from './types';
 
 const features: FeatureFlagMap = {
+    DUMMY_TEST_FLAG: null,
     ENABLE_ELEMENT_PATCH: null,
     ENABLE_FORCE_NATIVE_SHADOW_MODE_FOR_TEST: null,
     ENABLE_HMR: null,
diff --git a/packages/@lwc/features/src/types.ts b/packages/@lwc/features/src/types.ts
index bc6d7186c7..348e6f2155 100644
--- a/packages/@lwc/features/src/types.ts
+++ b/packages/@lwc/features/src/types.ts
@@ -16,6 +16,11 @@
 export type FeatureFlagValue = boolean | null;
 
 export interface FeatureFlagMap {
+    /**
+     * This is only used to test that feature flags are actually working
+     */
+    DUMMY_TEST_FLAG: FeatureFlagValue;
+
     /**
      * LWC engine flag to enable mixed shadow mode. Setting this flag to `true` enables usage of
      * native shadow DOM even when the synthetic shadow polyfill is applied.
diff --git a/packages/@lwc/synthetic-shadow/src/index.ts b/packages/@lwc/synthetic-shadow/src/index.ts
index cc64722ee7..135e45cd1a 100644
--- a/packages/@lwc/synthetic-shadow/src/index.ts
+++ b/packages/@lwc/synthetic-shadow/src/index.ts
@@ -39,3 +39,6 @@ import './faux-shadow/html-element';
 import './faux-shadow/slot';
 import './faux-shadow/portal';
 import './faux-shadow/shadow-token';
+
+// Tests
+import './testFeatureFlag';
diff --git a/packages/@lwc/synthetic-shadow/src/testFeatureFlag.ts b/packages/@lwc/synthetic-shadow/src/testFeatureFlag.ts
new file mode 100644
index 0000000000..a4fbab8348
--- /dev/null
+++ b/packages/@lwc/synthetic-shadow/src/testFeatureFlag.ts
@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2018, salesforce.com, inc.
+ * All rights reserved.
+ * SPDX-License-Identifier: MIT
+ * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
+ */
+
+import features from '@lwc/features';
+
+if (process.env.NODE_ENV !== 'production' && typeof window !== 'undefined') {
+    window.addEventListener('test-dummy-flag', () => {
+        let hasFlag = false;
+        if (features.DUMMY_TEST_FLAG) {
+            hasFlag = true;
+        }
+
+        window.dispatchEvent(
+            new CustomEvent('has-dummy-flag', {
+                detail: {
+                    package: '@lwc/synthetic-shadow',
+                    hasFlag,
+                },
+            })
+        );
+    });
+}
diff --git a/packages/integration-karma/scripts/karma-plugins/env.js b/packages/integration-karma/scripts/karma-plugins/env.js
index 37048e3696..833a1c385f 100644
--- a/packages/integration-karma/scripts/karma-plugins/env.js
+++ b/packages/integration-karma/scripts/karma-plugins/env.js
@@ -42,6 +42,7 @@ function createEnvFile() {
                 MIXED_SHADOW: ${FORCE_NATIVE_SHADOW_MODE_FOR_TEST},
                 NATIVE_SHADOW: ${!SYNTHETIC_SHADOW_ENABLED || FORCE_NATIVE_SHADOW_MODE_FOR_TEST},
                 NATIVE_SHADOW_ROOT_DEFINED: typeof ShadowRoot !== 'undefined',
+                SYNTHETIC_SHADOW_ENABLED: ${SYNTHETIC_SHADOW_ENABLED},
                 LWC_VERSION: ${JSON.stringify(LWC_VERSION)}
             }
         };
diff --git a/packages/integration-karma/test/features/index.spec.js b/packages/integration-karma/test/features/index.spec.js
new file mode 100644
index 0000000000..caf0706547
--- /dev/null
+++ b/packages/integration-karma/test/features/index.spec.js
@@ -0,0 +1,46 @@
+import { setFeatureFlagForTest } from 'lwc';
+
+describe('feature flags', () => {
+    let packages;
+
+    const onHasDummyFlag = (event) => {
+        if (event.detail.hasFlag) {
+            packages.push(event.detail.package);
+        }
+    };
+
+    beforeEach(() => {
+        packages = [];
+        window.addEventListener('has-dummy-flag', onHasDummyFlag);
+    });
+
+    afterEach(() => {
+        window.removeEventListener('has-dummy-flag', onHasDummyFlag);
+    });
+
+    describe('flag enabled', () => {
+        beforeEach(() => {
+            setFeatureFlagForTest('DUMMY_TEST_FLAG', true);
+        });
+
+        afterEach(() => {
+            setFeatureFlagForTest('DUMMY_TEST_FLAG', false);
+        });
+
+        it('works', () => {
+            window.dispatchEvent(new CustomEvent('test-dummy-flag'));
+            const expected = ['@lwc/engine-core', '@lwc/engine-dom'];
+            if (process.env.SYNTHETIC_SHADOW_ENABLED) {
+                expected.push('@lwc/synthetic-shadow');
+            }
+            expect(packages.sort()).toEqual(expected);
+        });
+    });
+
+    describe('flag disabled', () => {
+        it('works', () => {
+            window.dispatchEvent(new CustomEvent('test-dummy-flag'));
+            expect(packages.sort()).toEqual([]);
+        });
+    });
+});
diff --git a/scripts/rollup/lwcFeatures.js b/scripts/rollup/lwcFeatures.js
index ef4e2aa71f..39015049ec 100644
--- a/scripts/rollup/lwcFeatures.js
+++ b/scripts/rollup/lwcFeatures.js
@@ -14,8 +14,13 @@ module.exports = function lwcFeatures() {
     return {
         id: 'rollup-plugin-lwc-features',
         transform(source, id) {
-            if (id.includes('/node_modules/') || !source.includes('@lwc/features')) {
+            if (
+                id.includes('/node_modules/') ||
+                id.includes('/dist/') ||
+                !source.includes('@lwc/features')
+            ) {
                 // Skip 3rd-party files and files that don't mention @lwc/features
+                // Also skip /dist/ files because these have presumably already been run through lwcFeatures
                 return null;
             }
             return babel.transform(source, {