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(portalContainer.innerHTML).toBe('');
    expect(container.innerHTML).toBe('');
  });
  
  it('should support unstable_createPortal alias', () => {
    const portalContainer = document.createElement('div');
    expect(() =>
      ReactDOM.render(
        <div>
          {ReactDOM.unstable_createPortal(<div>portal</div>, portalContainer)}
        </div>,
        container,
      ),
    ).toLowPriorityWarnDev(
      'The ReactDOM.unstable_createPortal() alias has been deprecated, ' +
        'and will be removed in React 17+. Update your code to use ' +
        'ReactDOM.createPortal() instead. It has the exact same API, ' +
        'but without the "unstable_" prefix.',
    );
    expect(portalContainer.innerHTML).toBe('<div>portal</div>');
    expect(container.innerHTML).toBe('<div></div>');
    ReactDOM.unmountComponentAtNode(container);
    expect(portalContainer.innerHTML).toBe('');
    expect(container.innerHTML).toBe('');
  });
  it('should render many portals', () => {
    const portalContainer1 = document.createElement('div');
    const portalContainer2 = document.createElement('div');
    const ops = [];
    class Child extends React.Component {