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.
static contextTypes = {
          foo: PropTypes.string.isRequired,
          getFoo: PropTypes.func.isRequired,
        };
        render() {
          return <div>{this.context.foo + '-' + this.context.getFoo()}</div>;
        }
      }
      class Parent extends React.Component {
        static childContextTypes = {
          foo: PropTypes.string.isRequired,
          getFoo: PropTypes.func.isRequired,
        };
        getChildContext() {
          return {
            foo: this.props.bar,
            getFoo: () => this.props.bar,
          };
        }
        render() {