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 ReactDOMFiberEntry
 * @flow
 */
'use strict';
import type {Fiber} from 'ReactFiber';
import type {ReactNodeList} from 'ReactTypes';
var ExecutionEnvironment = require('fbjs/lib/ExecutionEnvironment');
var ReactBrowserEventEmitter = require('ReactBrowserEventEmitter');
var ReactControlledComponent = require('ReactControlledComponent');
var ReactDOMComponentTree = require('ReactDOMComponentTree');
var ReactFeatureFlags = require('ReactFeatureFlags');
var ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
var ReactDOMFiberComponent = require('ReactDOMFiberComponent');
var ReactDOMFrameScheduling = require('ReactDOMFrameScheduling');
var ReactGenericBatching = require('ReactGenericBatching');
var ReactFiberReconciler = require('ReactFiberReconciler');