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.
ops.length = 0;
      ReactDOM.unmountComponentAtNode(container);
      expect(portalContainer1.innerHTML).toBe('');
      expect(portalContainer2.innerHTML).toBe('');
      expect(container.innerHTML).toBe('');
      expect(ops).toEqual([
        'Parent:b componentWillUnmount',
        'normal[0]:b componentWillUnmount',
        'portal1[0]:b componentWillUnmount',
        'normal[1]:b componentWillUnmount',
        'portal2[0]:b componentWillUnmount',
        'portal2[1]:b componentWillUnmount',
      ]);
    });
    it('should render nested portals', () => {
      var portalContainer1 = document.createElement('div');
      var portalContainer2 = document.createElement('div');
      var portalContainer3 = document.createElement('div');
      ReactDOM.render(
        [
          <div key="a">normal[0]</div>,
          ReactDOM.unstable_createPortal(
            [
              <div key="b">portal1[0]</div>,
              ReactDOM.unstable_createPortal(
                <div key="c">portal2[0]</div>,
                portalContainer2,
              ),
              ReactDOM.unstable_createPortal(
                <div key="d">portal3[0]</div>,