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.
if (from) {
          ReactTestUtils.simulateNativeEventOnNode('topMouseOut', from, {
            target: from,
            relatedTarget: to,
          });
        }
        if (to) {
          ReactTestUtils.simulateNativeEventOnNode('topMouseOver', to, {
            target: to,
            relatedTarget: from,
          });
        }
      }
      ReactDOM.render(
        <div>
          <div
            onMouseEnter={() => ops.push('enter parent')}
            onMouseLeave={() => ops.push('leave parent')}>
            <div ref={n => (firstTarget = n)} />
            {ReactDOM.unstable_createPortal(
              <div
                onMouseEnter={() => ops.push('enter portal')}
                onMouseLeave={() => ops.push('leave portal')}