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.
static childContextTypes = {
          foo: PropTypes.string.isRequired,
          getFoo: PropTypes.func.isRequired,
        };
        state = {
          bar: 'initial',
        };
        getChildContext() {
          return {
            foo: this.state.bar,
            getFoo: () => this.state.bar,
          };
        }
        render() {
          return ReactDOM.unstable_createPortal(<Component />, portalContainer);
        }
      }
      var instance = ReactDOM.render(<Parent />, container);
      expect(portalContainer.innerHTML).toBe('<div>initial-initial</div>');
      expect(container.innerHTML).toBe('');
      instance.setState({bar: 'changed'});
      expect(portalContainer.innerHTML).toBe('<div>changed-changed</div>');
      expect(container.innerHTML).toBe('');
    });
    it('should update portal context if it changes due to re-render', () => {
      var portalContainer = document.createElement('div');
      class Component extends React.Component {