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 
if (from) {
        ReactTestUtils.SimulateNative.mouseOut(from, {
          relatedTarget: to,
        });
      }
      if (to) {
        ReactTestUtils.SimulateNative.mouseOver(to, {
          relatedTarget: from,
        });
      }
    }
    ReactDOM.render(
      <div>
        <div
          onMouseEnter={() => ops.push('enter parent')}
          onMouseLeave={() => ops.push('leave parent')}>
          <div ref={n => (firstTarget = n)} />
          {ReactDOM.createPortal(
            <div
              onMouseEnter={() => ops.push('enter portal')}
              onMouseLeave={() => ops.push('leave portal')}
              ref={n => (secondTarget = n)}>
              portal
            </div>,
            portalContainer,
          )}