Eagerly initialize an mutable object for instance.refs (facebook#25696)

This micro-optimization never made sense and less so now that they're
rare.

This still initializes the class with a shared immutable object in the
constructor - which is also what createClass() does.

Then we override it during mount. This is done in case someone messes up
the initialization of the super() constructor for example, which was
more common in polyfills.

This change means that if a ref is initialized during the constructor
itself it wouldn't be lazily initialized but that's not user code that
does it, it's React so that shouldn't happen.

This makes string refs codemoddable as described in.
facebook#25334

packages/react-reconciler/src/ReactChildFiber.new.js

@@ -39,7 +39,6 @@ import {
   createFiberFromText,
   createFiberFromPortal,
 } from './ReactFiber.new';
-import {emptyRefsObject} from './ReactFiberClassComponent.new';
 import {isCompatibleFamilyForHotReloading} from './ReactFiberHotReloading.new';
 import {StrictLegacyMode} from './ReactTypeOfMode';
 import {getIsHydrating} from './ReactFiberHydrationContext.new';
@@ -192,11 +191,7 @@ function coerceRef(
         return current.ref;
       }
       const ref = function(value) {
-        let refs = resolvedInst.refs;
-        if (refs === emptyRefsObject) {
-          // This is a lazy pooled frozen object, so we need to initialize.
-          refs = resolvedInst.refs = {};
-        }
+        const refs = resolvedInst.refs;
         if (value === null) {
           delete refs[stringRef];
         } else {

packages/react-reconciler/src/ReactChildFiber.old.js

@@ -39,7 +39,6 @@ import {
   createFiberFromText,
   createFiberFromPortal,
 } from './ReactFiber.old';
-import {emptyRefsObject} from './ReactFiberClassComponent.old';
 import {isCompatibleFamilyForHotReloading} from './ReactFiberHotReloading.old';
 import {StrictLegacyMode} from './ReactTypeOfMode';
 import {getIsHydrating} from './ReactFiberHydrationContext.old';
@@ -192,11 +191,7 @@ function coerceRef(
         return current.ref;
       }
       const ref = function(value) {
-        let refs = resolvedInst.refs;
-        if (refs === emptyRefsObject) {
-          // This is a lazy pooled frozen object, so we need to initialize.
-          refs = resolvedInst.refs = {};
-        }
+        const refs = resolvedInst.refs;
         if (value === null) {
           delete refs[stringRef];
         } else {

packages/react-reconciler/src/ReactFiberClassComponent.new.js

@@ -12,7 +12,6 @@ import type {Lanes} from './ReactFiberLane.new';
 import type {UpdateQueue} from './ReactFiberClassUpdateQueue.new';
 import type {Flags} from './ReactFiberFlags';
 
-import * as React from 'react';
 import {
   LayoutStatic,
   Update,
@@ -80,12 +79,6 @@ import {
 
 const fakeInternalInstance = {};
 
-// React.Component uses a shared frozen object by default.
-// We'll use it to determine whether we need to initialize legacy refs.
-export const emptyRefsObject: $FlowFixMe = React.Component
-  ? new React.Component().refs
-  : {};
-
 let didWarnAboutStateAssignmentForComponent;
 let didWarnAboutUninitializedState;
 let didWarnAboutGetSnapshotBeforeUpdateWithoutDidUpdate;
@@ -836,7 +829,7 @@ function mountClassInstance(
   const instance = workInProgress.stateNode;
   instance.props = newProps;
   instance.state = workInProgress.memoizedState;
-  instance.refs = emptyRefsObject;
+  instance.refs = {};
 
   initializeUpdateQueue(workInProgress);
 

packages/react-reconciler/src/ReactFiberClassComponent.old.js

@@ -12,7 +12,6 @@ import type {Lanes} from './ReactFiberLane.old';
 import type {UpdateQueue} from './ReactFiberClassUpdateQueue.old';
 import type {Flags} from './ReactFiberFlags';
 
-import * as React from 'react';
 import {
   LayoutStatic,
   Update,
@@ -80,12 +79,6 @@ import {
 
 const fakeInternalInstance = {};
 
-// React.Component uses a shared frozen object by default.
-// We'll use it to determine whether we need to initialize legacy refs.
-export const emptyRefsObject: $FlowFixMe = React.Component
-  ? new React.Component().refs
-  : {};
-
 let didWarnAboutStateAssignmentForComponent;
 let didWarnAboutUninitializedState;
 let didWarnAboutGetSnapshotBeforeUpdateWithoutDidUpdate;
@@ -836,7 +829,7 @@ function mountClassInstance(
   const instance = workInProgress.stateNode;
   instance.props = newProps;
   instance.state = workInProgress.memoizedState;
-  instance.refs = emptyRefsObject;
+  instance.refs = {};
 
   initializeUpdateQueue(workInProgress);