WIP Fix issues/bugs introduced by merge conflict resolution

**what is the change?:**
NOTE: There are still bugs here, we will need at least a few more fixes.

A couple of bugs and holes were introduced when cherry-picking #9232 onto the 15.6 branch. This fixes them.

**why make this change?:**
To keep tests passing and get this change working.

**test plan:**
`yarn test`

**issue:**
#9398

src/isomorphic/React.js

@@ -12,15 +12,16 @@
 'use strict';
 
 var ReactChildren = require('ReactChildren');
+var ReactClass = require('ReactClass');
 var ReactComponent = require('ReactComponent');
 var ReactPureComponent = require('ReactPureComponent');
-var ReactClass = require('ReactClass');
 var ReactDOMFactories = require('ReactDOMFactories');
 var ReactElement = require('ReactElement');
 var ReactPropTypes = require('ReactPropTypes');
 var ReactVersion = require('ReactVersion');
 
 var onlyChild = require('onlyChild');
+var warning = require('warning');
 
 var createElement = ReactElement.createElement;
 var createFactory = ReactElement.createFactory;
@@ -55,16 +56,6 @@ if (__DEV__) {
     warnedForSpread = true;
     return Object.assign.apply(null, arguments);
   };
-
-  createMixin = function(mixin) {
-    lowPriorityWarning(
-      warnedForCreateMixin,
-      'React.createMixin is deprecated and should not be used. You ' +
-        'can use this mixin directly instead.',
-    );
-    warnedForCreateMixin = true;
-    return mixin;
-  };
 }
 
 var React = {
@@ -102,8 +93,9 @@ var React = {
   __spread: __spread,
 };
 
-// TODO: Fix tests so that this deprecation warning doesn't cause failures.
 if (__DEV__) {
+  let warnedForCreateMixin = false;
+  let warnedForCreateClass = false;
   if (canDefineProperty) {
     Object.defineProperty(React, 'PropTypes', {
       get() {
@@ -127,7 +119,7 @@ if (__DEV__) {
             'drop-in replacement.',
         );
         didWarnPropTypesDeprecated = true;
-        return ReactPropTypes;
+        return ReactClass.createClass;
       },
     });
   }