Enable opt-in to async mode (#10535)

When the feature flag enableAsyncSubtreeAPI is true,
React.unstable_AsyncComponent creates an async subtree. When it's false,
it behaves like a normal, sync React component. We use this flag in www
as a fail safe so that if we ship an async bug, we can set the flag to
false and revert back to sync mode.

For open source, we should enable the feature flag.

src/renderers/dom/fiber/__tests__/ReactDOMFiberAsync-test.js

@@ -26,18 +26,28 @@ describe('ReactDOMFiberAsync', () => {
   });
 
   if (ReactDOMFeatureFlags.useFiber) {
-    it('renders synchronously when feature flag is disabled', () => {
-      ReactDOM.render(
-        <AsyncComponent><div>Hi</div></AsyncComponent>,
-        container,
-      );
-      expect(container.textContent).toEqual('Hi');
+    describe('with feature flag disabled', () => {
+      beforeEach(() => {
+        jest.resetModules();
+        ReactFeatureFlags = require('ReactFeatureFlags');
+        container = document.createElement('div');
+        ReactFeatureFlags.enableAsyncSubtreeAPI = false;
+        ReactDOM = require('react-dom');
+      });
 
-      ReactDOM.render(
-        <AsyncComponent><div>Bye</div></AsyncComponent>,
-        container,
-      );
-      expect(container.textContent).toEqual('Bye');
+      it('renders synchronously', () => {
+        ReactDOM.render(
+          <AsyncComponent><div>Hi</div></AsyncComponent>,
+          container,
+        );
+        expect(container.textContent).toEqual('Hi');
+
+        ReactDOM.render(
+          <AsyncComponent><div>Bye</div></AsyncComponent>,
+          container,
+        );
+        expect(container.textContent).toEqual('Bye');
+      });
     });
 
     describe('with feature flag enabled', () => {

src/renderers/shared/utils/ReactFeatureFlags.js

@@ -14,7 +14,7 @@
 
 var ReactFeatureFlags = {
   disableNewFiberFeatures: false,
-  enableAsyncSubtreeAPI: false,
+  enableAsyncSubtreeAPI: true,
 };
 
 module.exports = ReactFeatureFlags;