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 
);
    expect(ops).toEqual([
      'normal[0]:a componentDidMount',
      'portal1[0]:a componentDidMount',
      'normal[1]:a componentDidMount',
      'portal2[0]:a componentDidMount',
      'portal2[1]:a componentDidMount',
      'Parent:a componentDidMount',
    ]);
    ops.length = 0;
    ReactDOM.render(<Parent step="b" />, container);
    expect(portalContainer1.innerHTML).toBe('<div>portal1[0]:b</div>');
    expect(portalContainer2.innerHTML).toBe(
      '<div>portal2[0]:b</div><div>portal2[1]:b</div>',
    );
    expect(container.innerHTML).toBe(
      '<div>normal[0]:b</div><div>normal[1]:b</div>',
    );
    expect(ops).toEqual([
      'normal[0]:b componentDidUpdate',
      'portal1[0]:b componentDidUpdate',
      'normal[1]:b componentDidUpdate',
      'portal2[0]:b componentDidUpdate',
      'portal2[1]:b componentDidUpdate',
      'Parent:b componentDidUpdate',
    ]);
    ops.length = 0;
    ReactDOM.unmountComponentAtNode(container);
    expect(portalContainer1.innerHTML).toBe('');
    expect(portalContainer2.innerHTML).toBe('');