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 
</div>
        <div ref={n => (thirdTarget = n)} />
      </div>,
      container,
    );
    simulateMouseMove(null, firstTarget);
    expect(ops).toEqual(['enter parent']);
    ops = [];
    simulateMouseMove(firstTarget, secondTarget);
    expect(ops).toEqual([
      
      'enter portal',
    ]);
    ops = [];
    simulateMouseMove(secondTarget, thirdTarget);
    expect(ops).toEqual([
      'leave portal',
      'leave parent', 
    ]);
  });
  it('should throw on bad createPortal argument', () => {
    expect(() => {
      ReactDOM.createPortal(<div>portal</div>, null);
    }).toThrow('Target container is not a DOM element.');
    expect(() => {
      ReactDOM.createPortal(<div>portal</div>, document.createTextNode('hi'));
    }).toThrow('Target container is not a DOM element.');
  });
  it('should warn for non-functional event listeners', () => {
    class Example extends React.Component {