Enable enableUnifiedSyncLane (facebook#27646)

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## Summary

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Explain the **motivation** for making this change. What existing problem
does the pull request solve?
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The flag has been tested internally on WWW, should be good to set to
true for OSS. Added a dynamic flag for fb RN.

## How did you test this change?

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yarn test

packages/shared/ReactFeatureFlags.js

@@ -151,7 +151,7 @@ export const enableUseRefAccessWarning = false;
 // Enables time slicing for updates that aren't wrapped in startTransition.
 export const forceConcurrentByDefaultForTesting = false;
 
-export const enableUnifiedSyncLane = __EXPERIMENTAL__;
+export const enableUnifiedSyncLane = true;
 
 // Adds an opt-in to time slicing for updates that aren't wrapped in startTransition.
 export const allowConcurrentByDefault = false;

packages/shared/forks/ReactFeatureFlags.native-fb-dynamic.js

@@ -22,6 +22,7 @@ import typeof * as DynamicFlagsType from 'ReactNativeInternalFeatureFlags';
 
 export const enableUseRefAccessWarning = __VARIANT__;
 export const enableDeferRootSchedulingToMicrotask = __VARIANT__;
+export const enableUnifiedSyncLane = __VARIANT__;
 export const alwaysThrottleRetries = __VARIANT__;
 export const useMicrotasksForSchedulingInFabric = __VARIANT__;
 export const passChildrenWhenCloningPersistedNodes = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -20,6 +20,7 @@ import * as dynamicFlags from 'ReactNativeInternalFeatureFlags';
 export const {
   enableUseRefAccessWarning,
   enableDeferRootSchedulingToMicrotask,
+  enableUnifiedSyncLane,
   alwaysThrottleRetries,
   useMicrotasksForSchedulingInFabric,
   passChildrenWhenCloningPersistedNodes,
@@ -72,7 +73,6 @@ export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableLegacyHidden = true;
 export const forceConcurrentByDefaultForTesting = false;
-export const enableUnifiedSyncLane = false;
 export const allowConcurrentByDefault = true;
 export const enableCustomElementPropertySupport = false;
 

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -57,7 +57,7 @@ export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableLegacyHidden = false;
 export const forceConcurrentByDefaultForTesting = false;
-export const enableUnifiedSyncLane = false;
+export const enableUnifiedSyncLane = true;
 export const allowConcurrentByDefault = false;
 export const enableCustomElementPropertySupport = false;
 

packages/shared/forks/ReactFeatureFlags.test-renderer.native.js

@@ -56,7 +56,7 @@ export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableLegacyHidden = false;
 export const forceConcurrentByDefaultForTesting = false;
-export const enableUnifiedSyncLane = false;
+export const enableUnifiedSyncLane = true;
 export const allowConcurrentByDefault = true;
 
 export const consoleManagedByDevToolsDuringStrictMode = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -57,7 +57,7 @@ export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableLegacyHidden = false;
 export const forceConcurrentByDefaultForTesting = false;
-export const enableUnifiedSyncLane = false;
+export const enableUnifiedSyncLane = true;
 export const allowConcurrentByDefault = true;
 export const enableCustomElementPropertySupport = false;
 

scripts/flow/xplat.js

@@ -10,6 +10,7 @@
 declare module 'ReactNativeInternalFeatureFlags' {
   declare export var enableUseRefAccessWarning: boolean;
   declare export var enableDeferRootSchedulingToMicrotask: boolean;
+  declare export var enableUnifiedSyncLane: boolean;
   declare export var alwaysThrottleRetries: boolean;
   declare export var useMicrotasksForSchedulingInFabric: boolean;
   declare export var passChildrenWhenCloningPersistedNodes: boolean;