Get rid of the previousStyleCopy

We have warned about not mutating styles already. At this point we can just
freeze the style object in DEV.

That way we can read it from the previous props object without storing
another copy of it.

Also delete the associated warning.

src/renderers/dom/fiber/ReactDOMFiberComponent.js

@@ -99,43 +99,6 @@ function friendlyStringify(obj) {
   return String(obj);
 }
 
-var styleMutationWarning = {};
-
-function checkAndWarnForMutatedStyle(style1, style2, component) {
-  if (style1 == null || style2 == null) {
-    return;
-  }
-  if (shallowEqual(style1, style2)) {
-    return;
-  }
-
-  var componentName = component._tag;
-  var owner = component._currentElement._owner;
-  var ownerName;
-  if (owner) {
-    ownerName = owner.getName();
-  }
-
-  var hash = ownerName + '|' + componentName;
-
-  if (styleMutationWarning.hasOwnProperty(hash)) {
-    return;
-  }
-
-  styleMutationWarning[hash] = true;
-
-  warning(
-    false,
-    '`%s` was passed a style object that has previously been mutated. ' +
-    'Mutating `style` is deprecated. Consider cloning it beforehand. Check ' +
-    'the `render` %s. Previous style: %s. Mutated style: %s.',
-    componentName,
-    owner ? 'of `' + ownerName + '`' : 'using <' + componentName + '>',
-    friendlyStringify(style1),
-    friendlyStringify(style2)
-  );
-}
-
 /**
  * @param {object} component
  * @param {?object} props
@@ -526,14 +489,13 @@ function updateDOMProperties(
       continue;
     }
     if (propKey === STYLE) {
-      var lastStyle = workInProgress._previousStyleCopy;
+      var lastStyle = lastProps[propKey];
       for (styleName in lastStyle) {
         if (lastStyle.hasOwnProperty(styleName)) {
           styleUpdates = styleUpdates || {};
           styleUpdates[styleName] = '';
         }
       }
-      workInProgress._previousStyleCopy = null;
     } else if (registrationNameModules.hasOwnProperty(propKey)) {
       // Do nothing for deleted listeners.
     } else if (isCustomComponent(workInProgress._tag, lastProps)) {
@@ -552,26 +514,19 @@ function updateDOMProperties(
   for (propKey in nextProps) {
     var nextProp = nextProps[propKey];
     var lastProp =
-      propKey === STYLE ? workInProgress._previousStyleCopy :
       lastProps != null ? lastProps[propKey] : undefined;
     if (!nextProps.hasOwnProperty(propKey) ||
         nextProp === lastProp ||
         nextProp == null && lastProp == null) {
       continue;
     }
     if (propKey === STYLE) {
-      if (nextProp) {
-        if (__DEV__) {
-          checkAndWarnForMutatedStyle(
-            workInProgress._previousStyleCopy,
-            workInProgress._previousStyle,
-            workInProgress
-          );
-          workInProgress._previousStyle = nextProp;
+      if (__DEV__) {
+        if (nextProp) {
+          // Freeze the next style object so that we can assume it won't be
+          // mutated. We have already warned for this in the past.
+          Object.freeze(nextProp);
         }
-        nextProp = workInProgress._previousStyleCopy = Object.assign({}, nextProp);
-      } else {
-        workInProgress._previousStyleCopy = null;
       }
       if (lastProp) {
         // Unset styles on `lastProp` but not on `nextProp`.