Enable usage of string-refs codemod

Author: eps1lon

packages/react/src/ReactBaseClasses.js

@@ -7,9 +7,14 @@
 
 import ReactNoopUpdateQueue from './ReactNoopUpdateQueue';
 import assign from 'shared/assign';
+import {warnAboutStringRefs} from 'shared/ReactFeatureFlags';
 
 const emptyObject = {};
-if (__DEV__) {
+if (
+  __DEV__ &&
+  // For string refs we recommend the `string-refs` codemod that requires unsealed `this.refs`
+  !warnAboutStringRefs
+) {
   Object.freeze(emptyObject);
 }