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 
const textContent = 'Hello world';
    const iframe = document.createElement('iframe');
    document.body.appendChild(iframe);
    const iframeDocument = iframe.contentDocument;
    iframeDocument.write(
      '<!DOCTYPE html><html><head></head><body><div></div></body></html>',
    );
    iframeDocument.close();
    const iframeContainer = iframeDocument.body.firstChild;
    let actualDocument;
    let textNode;
    spyOnDevAndProd(iframeContainer, 'appendChild').and.callFake(node => {
      actualDocument = node.ownerDocument;
      textNode = node;
    });
    ReactDOM.render(textContent, iframeContainer);
    expect(textNode.textContent).toBe(textContent);
    expect(actualDocument).not.toBe(document);
    expect(actualDocument).toBe(iframeDocument);
    expect(iframeContainer.appendChild).toHaveBeenCalledTimes(1);
  });
  it('should mount into a document fragment', () => {
    const fragment = document.createDocumentFragment();
    ReactDOM.render(<div>foo</div>, fragment);
    expect(container.innerHTML).toBe('');
    container.appendChild(fragment);
    expect(container.innerHTML).toBe('<div>foo</div>');
  });
});