Ship diffInCommitPhase (facebook#27409)

Performance tests at Meta showed neutral results.

Author: sophiebits

packages/react-art/src/ReactFiberConfigART.js

@@ -15,10 +15,8 @@ import {DefaultEventPriority} from 'react-reconciler/src/ReactEventPriorities';
 const pooledTransform = new Transform();
 
 const NO_CONTEXT = {};
-const UPDATE_SIGNAL = {};
 if (__DEV__) {
   Object.freeze(NO_CONTEXT);
-  Object.freeze(UPDATE_SIGNAL);
 }
 
 /** Helper Methods */
@@ -312,10 +310,6 @@ export function prepareForCommit() {
   return null;
 }
 
-export function prepareUpdate(domElement, type, oldProps, newProps) {
-  return UPDATE_SIGNAL;
-}
-
 export function resetAfterCommit() {
   // Noop
 }

packages/react-dom-bindings/src/client/CSSPropertyOperations.js

@@ -11,7 +11,6 @@ import hyphenateStyleName from '../shared/hyphenateStyleName';
 import warnValidStyle from '../shared/warnValidStyle';
 import isUnitlessNumber from '../shared/isUnitlessNumber';
 import {checkCSSPropertyStringCoercion} from 'shared/CheckStringCoercion';
-import {diffInCommitPhase} from 'shared/ReactFeatureFlags';
 
 /**
  * Operations for dealing with CSS properties.
@@ -126,7 +125,7 @@ export function setValueForStyles(node, styles, prevStyles) {
 
   const style = node.style;
 
-  if (diffInCommitPhase && prevStyles != null) {
+  if (prevStyles != null) {
     if (__DEV__) {
       validateShorthandPropertyCollisionInDev(prevStyles, styles);
     }
@@ -200,10 +199,7 @@ function expandShorthandMap(styles) {
  *   {font: 'foo', fontVariant: 'bar'} -> {font: 'foo'}
  *   becomes .style.fontVariant = ''
  */
-export function validateShorthandPropertyCollisionInDev(
-  prevStyles,
-  nextStyles,
-) {
+function validateShorthandPropertyCollisionInDev(prevStyles, nextStyles) {
   if (__DEV__) {
     if (!nextStyles) {
       return;