expose isHydrating (#16909)

expose isHydrating for FB

Author: lunaruan

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

@@ -8,6 +8,7 @@
  */
 
 import {findCurrentFiberUsingSlowPath} from 'react-reconciler/reflection';
+import {getIsHydrating} from 'react-reconciler/src/ReactFiberHydrationContext';
 import {get as getInstance} from 'shared/ReactInstanceMap';
 import {addUserTimingListener} from 'shared/ReactFeatureFlags';
 
@@ -33,6 +34,8 @@ Object.assign(
     },
     // Perf experiment
     addUserTimingListener,
+
+    getIsHydrating,
   },
 );
 

packages/react-reconciler/src/ReactFiberHydrationContext.js

@@ -484,6 +484,10 @@ function resetHydrationState(): void {
   isHydrating = false;
 }
 
+function getIsHydrating(): boolean {
+  return isHydrating;
+}
+
 export {
   warnIfHydrating,
   enterHydrationState,
@@ -494,4 +498,5 @@ export {
   prepareToHydrateHostTextInstance,
   prepareToHydrateHostSuspenseInstance,
   popHydrationState,
+  getIsHydrating,
 };