Move ReactElementValidator to __DEV__ block (#6830)

It saves some more bytes in production mode.
(cherry picked from commit d955ee9fae)

src/isomorphic/React.js

@@ -16,7 +16,6 @@ var ReactComponent = require('ReactComponent');
 var ReactClass = require('ReactClass');
 var ReactDOMFactories = require('ReactDOMFactories');
 var ReactElement = require('ReactElement');
-var ReactElementValidator = require('ReactElementValidator');
 var ReactPropTypes = require('ReactPropTypes');
 var ReactVersion = require('ReactVersion');
 
@@ -28,6 +27,7 @@ var createFactory = ReactElement.createFactory;
 var cloneElement = ReactElement.cloneElement;
 
 if (__DEV__) {
+  var ReactElementValidator = require('ReactElementValidator');
   createElement = ReactElementValidator.createElement;
   createFactory = ReactElementValidator.createFactory;
   cloneElement = ReactElementValidator.cloneElement;

src/isomorphic/classic/element/ReactDOMFactories.js

@@ -12,7 +12,6 @@
 'use strict';
 
 var ReactElement = require('ReactElement');
-var ReactElementValidator = require('ReactElementValidator');
 
 var mapObject = require('mapObject');
 
@@ -24,6 +23,7 @@ var mapObject = require('mapObject');
  */
 function createDOMFactory(tag) {
   if (__DEV__) {
+    var ReactElementValidator = require('ReactElementValidator');
     return ReactElementValidator.createFactory(tag);
   }
   return ReactElement.createFactory(tag);