Revert "Support writing to this.refs from userspace" (facebook#28877)

Reverts facebook#28867

It broke some tests, reverting until we figure out why to avoid having
too much delay in the sync.

packages/react-reconciler/src/ReactFiberClassComponent.js

@@ -819,6 +819,7 @@ function mountClassInstance(
   const instance = workInProgress.stateNode;
   instance.props = newProps;
   instance.state = workInProgress.memoizedState;
+  instance.refs = {};
 
   initializeUpdateQueue(workInProgress);
 

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

@@ -138,28 +138,4 @@ describe('ReactFiberRefs', () => {
     );
     expect(refProp).toBe('child');
   });
-
-  test('strings refs can be codemodded to callback refs', async () => {
-    let app;
-    class App extends React.Component {
-      render() {
-        app = this;
-        return (
-          <div
-            prop="Hello!"
-            ref={el => {
-              // `refs` used to be a shared frozen object unless/until a string
-              // ref attached by the reconciler, but it's not anymore so that we
-              // can codemod string refs to userspace callback refs.
-              this.refs.div = el;
-            }}
-          />
-        );
-      }
-    }
-
-    const root = ReactNoop.createRoot();
-    await act(() => root.render(<App />));
-    expect(app.refs.div.prop).toBe('Hello!');
-  });
 });

packages/react/src/ReactBaseClasses.js

@@ -8,13 +8,19 @@
 import ReactNoopUpdateQueue from './ReactNoopUpdateQueue';
 import assign from 'shared/assign';
 
+const emptyObject = {};
+if (__DEV__) {
+  Object.freeze(emptyObject);
+}
+
 /**
  * Base class helpers for the updating state of a component.
  */
 function Component(props, context, updater) {
   this.props = props;
   this.context = context;
-  this.refs = {};
+  // If a component has string refs, we will assign a different object later.
+  this.refs = emptyObject;
   // We initialize the default updater but the real one gets injected by the
   // renderer.
   this.updater = updater || ReactNoopUpdateQueue;
@@ -127,7 +133,7 @@ function PureComponent(props, context, updater) {
   this.props = props;
   this.context = context;
   // If a component has string refs, we will assign a different object later.
-  this.refs = {};
+  this.refs = emptyObject;
   this.updater = updater || ReactNoopUpdateQueue;
 }