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.
ref={n => (secondTarget = n)}>
                portal
              </div>,
              portalContainer,
            )}
          </div>
          <div ref={n => (thirdTarget = n)} />
        </div>,
        container,
      );
      simulateMouseMove(null, firstTarget);
      expect(ops).toEqual(['enter parent']);
      ops = [];
      simulateMouseMove(firstTarget, secondTarget);
      expect(ops).toEqual([
        
        'enter portal',
      ]);
      ops = [];
      simulateMouseMove(secondTarget, thirdTarget);
      expect(ops).toEqual([
        'leave portal',
        'leave parent', 
      ]);
    });
    it('should not update event handlers until commit', () => {
      let ops = [];
      const handlerA = () => ops.push('A');
      const handlerB = () => ops.push('B');
      class Example extends React.Component {