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.
});
describe('disableNewFiberFeatures', () => {
  var container;
  var ReactFeatureFlags;
  beforeEach(() => {
    container = document.createElement('div');
    ReactFeatureFlags = require('ReactFeatureFlags');
    ReactFeatureFlags.disableNewFiberFeatures = true;
  });
  afterEach(() => {
    ReactFeatureFlags = require('ReactFeatureFlags');
    ReactFeatureFlags.disableNewFiberFeatures = false;
  });
  it('throws if non-element passed to top-level render', () => {
    const message = 'render(): Invalid component element.';
    expect(() => ReactDOM.render(null, container)).toThrow(message, container);
    expect(() => ReactDOM.render(undefined, container)).toThrow(
      message,
      container,
    );
    expect(() => ReactDOM.render(false, container)).toThrow(message, container);
    expect(() => ReactDOM.render('Hi', container)).toThrow(message, container);
    expect(() => ReactDOM.render(999, container)).toThrow(message, container);
    expect(() => ReactDOM.render([<div key="a" />], container)).toThrow(
      message,
      container,
    );
  });
  it('throws if something other than false, null, or an element is returned from render', () => {
    function Render(props) {