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.
expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'render(...): ' +
          'It looks like the React-rendered content of this container was ' +
          'removed without using React. This is not supported and will ' +
          'cause errors. Instead, call ReactDOM.unmountComponentAtNode ' +
          'to empty a container.',
      );
    });
    it('should warn when doing an update to a container manually cleared outside of React', () => {
      spyOn(console, 'error');
      
      ReactDOM.render(<div>foo</div>, container);
      ReactDOM.render(<div>bar</div>, container);
      expect(container.innerHTML).toBe('<div>bar</div>');
      
      container.innerHTML = '';
      ReactDOM.render(<div>baz</div>, container);
      
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'render(...): ' +
          'It looks like the React-rendered content of this container was ' +
          'removed without using React. This is not supported and will ' +
          'cause errors. Instead, call ReactDOM.unmountComponentAtNode ' +
          'to empty a container.',
      );
    });
  }