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(b.tagName).toBe('SPAN');
  });
  it('should bubble events from the portal to the parent', () => {
    const portalContainer = document.createElement('div');
    const ops = [];
    let portal = null;
    ReactDOM.render(
      <div onClick={() => ops.push('parent clicked')}>
        {ReactDOM.createPortal(
          <div
            onClick={() => ops.push('portal clicked')}
            ref={n => (portal = n)}>
            portal
          </div>,
          portalContainer,
        )}
      </div>,
      container,
    );
    expect(portal.tagName).toBe('DIV');
    ReactTestUtils.Simulate.click(portal);
    expect(ops).toEqual(['portal clicked', 'parent clicked']);
  });
  it('should not onMouseLeave when staying in the portal', () => {
    const portalContainer = document.createElement('div');
    let ops = [];
    let firstTarget = null;
    let secondTarget = null;
    let thirdTarget = null;
    function simulateMouseMove(from, to) {