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.
container = document.createElement('div');
    ReactFeatureFlags = require('ReactFeatureFlags');
    ReactFeatureFlags.disableNewFiberFeatures = false;
  });
  afterEach(() => {
    ReactFeatureFlags = require('ReactFeatureFlags');
    ReactFeatureFlags.disableNewFiberFeatures = true;
  });
  it('should render strings as children', () => {
    const Box = ({value}) => <div>{value}</div>;
    ReactDOM.render(<Box value="foo" />, container);
    expect(container.textContent).toEqual('foo');
  });
  it('should render numbers as children', () => {
    const Box = ({value}) => <div>{value}</div>;
    ReactDOM.render(<Box value={10} />, container);
    expect(container.textContent).toEqual('10');
  });
  it('should be called a callback argument', () => {
    
    let called = false;
    ReactDOM.render(<div>Foo</div>, container, () => (called = true));
    expect(called).toEqual(true);
    
    called = false;
    ReactDOM.render(<div>Foo</div>, container, () => (called = true));
    expect(called).toEqual(true);
  });
  it('should call a callback argument when the same element is re-rendered', () => {
    class Foo extends React.Component {