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.
| (Element & {
    _reactRootContainer: ?Object,
  })
  | (Document & {
    _reactRootContainer: ?Object,
  });
type Container = Element | Document;
type Props = {
  autoFocus?: boolean,
  children?: mixed,
  hidden?: boolean,
};
type Instance = Element;
type TextInstance = Text;
type HostContextDev = {
  namespace: string,
  ancestorInfo: mixed,
};
type HostContextProd = string;
type HostContext = HostContextDev | HostContextProd;
let eventsEnabled: ?boolean = null;
let selectionInformation: ?mixed = null;
/**
 * True if the supplied DOM node is a valid node element.
 *
 * @param {?DOMElement} node The candidate DOM node.
 * @return {boolean} True if the DOM is a valid DOM node.
 * @internal
 */
function isValidContainer(node) {