Put perf integration behind a feature flag (#11455)

* Enable User Timing API integration with a feature flag

* Expose a way to toggle user timing flag in www

* Update ReactNativeCSFeatureFlags.js

* Update ReactFeatureFlags.js

Author: gaearon

packages/react-cs-renderer/src/ReactNativeCSFeatureFlags.js

@@ -7,19 +7,27 @@
  * @flow
  */
 
+import invariant from 'fbjs/lib/invariant';
+
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
 import typeof * as CSFeatureFlagsType from './ReactNativeCSFeatureFlags';
 
 export const enableAsyncSubtreeAPI = true;
 export const enableAsyncSchedulingByDefaultInReactDOM = false;
 export const enableReactFragment = false;
 export const enableCreateRoot = false;
+export const enableUserTimingAPI = __DEV__;
 
 // React Native CS uses persistent reconciler.
 export const enableMutatingReconciler = false;
 export const enableNoopReconciler = false;
 export const enablePersistentReconciler = true;
 
+// Only used in www builds.
+export function addUserTimingListener() {
+  invariant(false, 'Not implemented.');
+}
+
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y=_X> = null;

packages/react-dom/src/client/ReactDOMFB.js

@@ -13,6 +13,7 @@ import * as ReactInstanceMap from 'shared/ReactInstanceMap';
 import * as ReactFiberErrorLogger
   from 'react-reconciler/src/ReactFiberErrorLogger';
 import ReactErrorUtils from 'shared/ReactErrorUtils';
+import {addUserTimingListener} from 'shared/ReactFeatureFlags';
 
 import ReactDOM from './ReactDOM';
 import * as ReactBrowserEventEmitter from '../events/ReactBrowserEventEmitter';
@@ -31,6 +32,8 @@ Object.assign(
     ReactInstanceMap,
     // Used by www msite:
     TapEventPlugin,
+    // Perf experiment
+    addUserTimingListener,
   },
 );