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 
portalContainer1,
        ),
        <div key="f">normal[1]</div>,
      ],
      container,
    );
    expect(portalContainer1.innerHTML).toBe(
      '<div>portal1[0]</div><div>portal1[1]</div>',
    );
    expect(portalContainer2.innerHTML).toBe('<div>portal2[0]</div>');
    expect(portalContainer3.innerHTML).toBe('<div>portal3[0]</div>');
    expect(container.innerHTML).toBe(
      '<div>normal[0]</div><div>normal[1]</div>',
    );
    ReactDOM.unmountComponentAtNode(container);
    expect(portalContainer1.innerHTML).toBe('');
    expect(portalContainer2.innerHTML).toBe('');
    expect(portalContainer3.innerHTML).toBe('');
    expect(container.innerHTML).toBe('');
  });
  it('should reconcile portal children', () => {
    const portalContainer = document.createElement('div');
    ReactDOM.render(
      <div>{ReactDOM.createPortal(<div>portal:1</div>, portalContainer)}</div>,
      container,
    );
    expect(portalContainer.innerHTML).toBe('<div>portal:1</div>');
    expect(container.innerHTML).toBe('<div></div>');
    ReactDOM.render(
      <div>{ReactDOM.createPortal(<div>portal:2</div>, portalContainer)}</div>,