https://codesandbox.io/s/40j50607y0

Rendering a component like:

```js
const Foo = () => <React.Fragment />
```

Throws the error:

> Foo(...): Nothing was returned from render. This usually means a return statement is missing. Or, to render nothing, return null.

The error is confusing because clearly `Foo` _is_ returning a value. I don't know if we want to support rendering empty fragments, but in any case this error is confusing.

If this isn't allowed it should explicitly call out that a component cannot return a fragment with no children.

cc @gaearon 
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,
      };
      state = {
        bar: 'initial',
      };
      getChildContext() {
        return {
          foo: this.state.bar,
          getFoo: () => this.state.bar,
        };
      }
      render() {