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,
      };
      render() {
        return <div>{this.context.foo}</div>;
      }
    }
    class Parent extends React.Component {
      static childContextTypes = {
        foo: PropTypes.string.isRequired,
      };
      getChildContext() {
        return {
          foo: 'bar',
        };
      }
      render() {
        return ReactDOM.createPortal(<Component />, portalContainer);
      }
    }
    ReactDOM.render(<Parent />, container);
    expect(container.innerHTML).toBe('');
    expect(portalContainer.innerHTML).toBe('<div>bar</div>');
  });
  it('should update portal context if it changes due to setState', () => {
    const portalContainer = document.createElement('div');
    class Component extends React.Component {