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.
'ReactDOM.unmountComponentAtNode to empty a container.',
        );
      }
    }
    const isRootRenderedBySomeReact = !!container._reactRootContainer;
    const rootEl = getReactRootElementInContainer(container);
    const hasNonRootReactChild = !!(rootEl &&
      ReactDOMComponentTree.getInstanceFromNode(rootEl));
    warning(
      !hasNonRootReactChild || isRootRenderedBySomeReact,
      'render(...): Replacing React-rendered children with a new root ' +
        'component. If you intended to update the children of this node, ' +
        'you should instead have the existing children update their state ' +
        'and render the new components instead of calling ReactDOM.render.',
    );
    warning(
      container.nodeType !== ELEMENT_NODE ||
        !((container: any): Element).tagName ||
        ((container: any): Element).tagName.toUpperCase() !== 'BODY',
      'render(): Rendering components directly into document.body is ' +
        'discouraged, since its children are often manipulated by third-party ' +
        'scripts and browser extensions. This may lead to subtle ' +
        'reconciliation issues. Try rendering into a container element created ' +
        'for your app.',
    );
  }
  let root = container._reactRootContainer;
  if (!root) {
    const shouldHydrate =
      forceHydrate || shouldHydrateDueToLegacyHeuristic(container);