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.
'parentComponent must be a valid React Component',
    );
    return renderSubtreeIntoContainer(
      parentComponent,
      element,
      containerNode,
      false,
      callback,
    );
  },
  unmountComponentAtNode(container: DOMContainer) {
    invariant(
      isValidContainer(container),
      'unmountComponentAtNode(...): Target container is not a DOM element.',
    );
    if (container._reactRootContainer) {
      if (__DEV__) {
        const rootEl = getReactRootElementInContainer(container);
        const renderedByDifferentReact =
          rootEl && !ReactDOMComponentTree.getInstanceFromNode(rootEl);
        warning(
          !renderedByDifferentReact,
          "unmountComponentAtNode(): The node you're attempting to unmount " +
            'was rendered by another copy of React.',
        );
      }
      
      DOMRenderer.unbatchedUpdates(() => {
        renderSubtreeIntoContainer(null, null, container, false, () => {
          container._reactRootContainer = null;