Currently if React is not loaded, ReactDOM shows this:

<img width="664" alt="screen shot 2017-08-08 at 3 36 52 pm" src="https://user-images.githubusercontent.com/810438/29077217-784c7624-7c4f-11e7-88bc-b055407523a9.png">

We should throw a nicer error message in the entry point I think.
/**
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule ReactDOMStackEntry
 */
'use strict';
var ReactDOMComponentTree = require('ReactDOMComponentTree');
var ReactGenericBatching = require('ReactGenericBatching');
var ReactMount = require('ReactMount');
var ReactUpdates = require('ReactUpdates');
var ReactReconciler = require('ReactReconciler');
var ReactVersion = require('ReactVersion');
var findDOMNode = require('findDOMNode');
var getHostComponentFromComposite = require('getHostComponentFromComposite');
if (__DEV__) {
  var warning = require('fbjs/lib/warning');
}
require('ReactDOMInjection');
require('ReactDOMClientInjection');
require('ReactDOMStackInjection');
var ReactDOMStack = {
  findDOMNode: findDOMNode,