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.
return props.children;
    }
    expect(() => ReactDOM.render(<Render>Hi</Render>, container)).toThrow(
      /You may have returned undefined/,
    );
    expect(() => ReactDOM.render(<Render>{999}</Render>, container)).toThrow(
      /You may have returned undefined/,
    );
    expect(() =>
      ReactDOM.render(<Render>[<div key="a" />]</Render>, container),
    ).toThrow(/You may have returned undefined/);
  });
  it('treats mocked render functions as if they return null', () => {
    class Mocked extends React.Component {}
    Mocked.prototype.render = jest.fn();
    ReactDOM.render(<Mocked />, container);
    expect(container.textContent).toEqual('');
  });
});