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 
} else {
          previousNewFiber.sibling = newFiber;
        }
        previousNewFiber = newFiber;
      }
    }
    if (shouldTrackSideEffects) {
      // 
      // 
      existingChildren.forEach(child => deleteChild(returnFiber, child));
    }
    return resultingFirstChild;
  }
  function reconcileChildrenIterator(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChildrenIterable: Iterable<*>,
    expirationTime: ExpirationTime,
  ): Fiber | null {
    // 
    // 
    const iteratorFn = getIteratorFn(newChildrenIterable);
    invariant(
      typeof iteratorFn === 'function',
      'An object is not an iterable. This error is likely caused by a bug in ' +
        'React. Please file an issue.',
    );
    if (__DEV__) {
      // 
      if ((newChildrenIterable: any).entries === iteratorFn) {