The cost of the separate file is not really warranted. Will use feature flag to scope build specific implementations

Author: gnoff

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 {