Split static and dynamic www feature flags (#11471)

scripts/rollup/shims/rollup/ReactFeatureFlags-www.js

@@ -10,18 +10,27 @@
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
 import typeof * as FeatureFlagsShimType from './ReactFeatureFlags-www';
 
-// Re-export all flags from the www version.
+// Re-export dynamic flags from the www version.
 export const {
-  enableAsyncSubtreeAPI,
   enableAsyncSchedulingByDefaultInReactDOM,
-  enableReactFragment,
-  enableCreateRoot,
-  // Reconciler flags
-  enableMutatingReconciler,
-  enableNoopReconciler,
-  enablePersistentReconciler,
 } = require('ReactFeatureFlags');
 
+// The rest of the flags are static for better dead code elimination.
+export const enableAsyncSubtreeAPI = true;
+export const enableReactFragment = false;
+export const enableCreateRoot = false;
+
+// The www bundles only use the mutating reconciler.
+export const enableMutatingReconciler = true;
+export const enableNoopReconciler = false;
+export const enablePersistentReconciler = false;
+
+// In www, we have experimental support for gathering data
+// from User Timing API calls in production. By default, we
+// only emit performance.mark/measure calls in __DEV__. But if
+// somebody calls addUserTimingListener() which is exposed as an
+// experimental FB-only export, we call performance.mark/measure
+// as long as there is more than a single listener.
 export let enableUserTimingAPI = __DEV__;
 
 let refCount = 0;
@@ -38,6 +47,9 @@ export function addUserTimingListener() {
   };
 }
 
+// The flag is intentionally updated in a timeout.
+// We don't support toggling it during reconciliation or
+// commit since that would cause mismatching user timing API calls.
 let timeout = null;
 function updateFlagOutsideOfReactCallStack() {
   if (!timeout) {