The cost of the separate file is not really warranted. Will use featu…

…re flag to scope build specific implementations

packages/react-dom/index.classic.fb.js

@@ -32,7 +32,7 @@ export {
  preinit,
  preinitModule,
  version,
-} from './src/client/ReactDOMFB';
+} from './src/client/ReactDOM';
 
 export {
  createRoot,

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

@@ -14,15 +14,18 @@ import type {
  CreateRootOptions,
 } from './ReactDOMRoot';
 
+import {disableLegacyMode} from 'shared/ReactFeatureFlags';
 import {
  createRoot as createRootImpl,
  hydrateRoot as hydrateRootImpl,
  isValidContainer,
 } from './ReactDOMRoot';
 import {createEventHandle} from 'react-dom-bindings/src/client/ReactDOMEventHandle';
-import {flushSync} from '../shared/ReactDOMFlushSync';
+import {flushSync as flushSyncIsomorphic} from '../shared/ReactDOMFlushSync';
 
 import {
+ flushSyncFromReconciler as flushSyncWithoutWarningIfAlreadyRendering,
+ isAlreadyRendering,
  injectIntoDevTools,
  findHostInstance,
 } from 'react-reconciler/src/ReactFiberReconciler';
@@ -120,6 +123,29 @@ function hydrateRoot(
  return hydrateRootImpl(container, initialChildren, options);
 }
 
+// Overload the definition to the two valid signatures.
+// Warning, this opts-out of checking the function body.
+declare function flushSyncFromReconciler<R>(fn: () => R): R;
+// eslint-disable-next-line no-redeclare
+declare function flushSyncFromReconciler(): void;
+// eslint-disable-next-line no-redeclare
+function flushSyncFromReconciler<R>(fn: (() => R) | void): R | void {
+ if (__DEV__) {
+ if (isAlreadyRendering()) {
+ console.error(
+ 'flushSync was called from inside a lifecycle method. React cannot ' +
+ 'flush when React is already rendering. Consider moving this call to ' +
+ 'a scheduler task or micro task.',
+ );
+ }
+ }
+ return flushSyncWithoutWarningIfAlreadyRendering(fn);
+}
+
+const flushSync: typeof flushSyncIsomorphic = disableLegacyMode
+ ? flushSyncIsomorphic
+ : flushSyncFromReconciler;
+
 function findDOMNode(
  componentOrElement: React$Component<any, any>,
 ): null | Element | Text {