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.
{ReactDOM.unstable_createPortal(['Bye', 'Hi'], portalContainer)}
        </div>,
        container,
      );
      expect(portalContainer.innerHTML).toBe('ByeHi');
      expect(container.innerHTML).toBe('<div></div>');
      ReactDOM.render(
        <div>
          {ReactDOM.unstable_createPortal(null, portalContainer)}
        </div>,
        container,
      );
      expect(portalContainer.innerHTML).toBe('');
      expect(container.innerHTML).toBe('<div></div>');
    });
    it('should keep track of namespace across portals (simple)', () => {
      assertNamespacesMatch(
        <svg {...expectSVG}>
          <image {...expectSVG} />
          {usePortal(<div {...expectHTML} />)}
          <image {...expectSVG} />
        </svg>,
      );
      assertNamespacesMatch(
        <math {...expectMath}>
          <mi {...expectMath} />
          {usePortal(<div {...expectHTML} />)}
          <mi {...expectMath} />
        </math>,
      );