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.
);
        } else if (element != null && typeof element.props !== 'undefined') {
          
          invariant(
            false,
            'ReactDOM.render(): Invalid component element. This may be ' +
              'caused by unintentionally loading two independent copies ' +
              'of React.',
          );
        } else {
          invariant(false, 'ReactDOM.render(): Invalid component element.');
        }
      }
    }
    return renderSubtreeIntoContainer(
      null,
      element,
      container,
      false,
      callback,
    );
  },
  unstable_renderSubtreeIntoContainer(
    parentComponent: ReactComponent<any, any, any>,
    element: ReactElement<any>,
    containerNode: DOMContainer,
    callback: ?Function,
  ) {
    invariant(
      parentComponent != null && ReactInstanceMap.has(parentComponent),