Export React.act from 18.3

Author: rickhanlonii

packages/react-reconciler/src/__tests__/ReactIsomorphicAct-test.js

@@ -20,7 +20,7 @@ describe('isomorphic act()', () => {
     ReactNoop = require('react-noop-renderer');
     DiscreteEventPriority = require('react-reconciler/constants')
       .DiscreteEventPriority;
-    act = React.unstable_act;
+    act = gate(flags => flags.variant) ? React.act : React.unstable_act;
   });
 
   beforeEach(() => {

packages/react/index.classic.fb.js

@@ -9,6 +9,7 @@
 
 export {
   __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED,
+  act,
   act as unstable_act,
   Children,
   Component,

packages/react/index.experimental.js

@@ -9,6 +9,7 @@
 
 export {
   __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED,
+  act,
   act as unstable_act,
   Children,
   Component,

packages/react/index.js

@@ -33,6 +33,7 @@ export type ChildrenArray<+T> = $ReadOnlyArray<ChildrenArray<T>> | T;
 // We can't use export * from in Flow for some reason.
 export {
   __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED,
+  act,
   act as unstable_act,
   Children,
   Component,

packages/react/index.modern.fb.js

@@ -9,6 +9,7 @@
 
 export {
   __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED,
+  act,
   act as unstable_act,
   Children,
   Component,

packages/react/index.stable.js

@@ -9,6 +9,7 @@
 
 export {
   __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED,
+  act,
   act as unstable_act,
   Children,
   Component,