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.
 *
 * @emails react-core
 */
'use strict';
var React = require('react');
var ReactDOM = require('react-dom');
var ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
var ReactTestUtils = require('react-dom/test-utils');
var PropTypes = require('prop-types');
describe('ReactDOMFiber', () => {
  var container;
  var ReactFeatureFlags;
  beforeEach(() => {