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.
var ReactInputSelection = require('ReactInputSelection');
var ReactInstanceMap = require('ReactInstanceMap');
var ReactPortal = require('ReactPortal');
var ReactVersion = require('ReactVersion');
var {isValidElement} = require('react');
var {injectInternals} = require('ReactFiberDevToolsHook');
var {
  ELEMENT_NODE,
  TEXT_NODE,
  COMMENT_NODE,
  DOCUMENT_NODE,
  DOCUMENT_FRAGMENT_NODE,
} = require('HTMLNodeType');
var {ROOT_ATTRIBUTE_NAME} = require('DOMProperty');
var findDOMNode = require('findDOMNode');
var invariant = require('fbjs/lib/invariant');
var {
  createElement,
  getChildNamespace,
  setInitialProperties,
  diffProperties,
  updateProperties,
  diffHydratedProperties,
  diffHydratedText,
  warnForDeletedHydratableElement,
  warnForDeletedHydratableText,
  warnForInsertedHydratedElement,
  warnForInsertedHydratedText,
} = ReactDOMFiberComponent;
var {precacheFiberNode, updateFiberProps} = ReactDOMComponentTree;