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 
const ref = function(value) {
        const refs = inst.refs === emptyObject ? (inst.refs = {}) : inst.refs;
        if (value === null) {
          delete refs[stringRef];
        } else {
          refs[stringRef] = value;
        }
      };
      ref._stringRef = stringRef;
      return ref;
    } else {
      invariant(
        typeof mixedRef === 'string',
        'Expected ref to be a function or a string.',
      );
      invariant(
        element._owner,
        'Element ref was specified as a string (%s) but no owner was set. This could happen for one of' +
          ' the following reasons:\n' +
          '1. You may be adding a ref to a functional component\n' +
          "2. You may be adding a ref to a component that was not created inside a component's render method\n" +
          '3. You have multiple copies of React loaded\n' +
          'See https://fb.me/react-refs-must-have-owner for more information.',
        mixedRef,
      );
    }
  }
  return mixedRef;
}
function throwOnInvalidObjectType(returnFiber: Fiber, newChild: Object) {