We used to throw if event listener is not a function in Stack:

```js
ReactDOM.render(
  <h1 onClick={{}}>Hello World!</h1>,
  document.getElementById('container')
);
```

<img width="565" alt="screen shot 2017-08-08 at 3 41 08 pm" src="https://user-images.githubusercontent.com/810438/29077502-2646412e-7c50-11e7-8c99-3b7f6c0a2589.png">

Fiber silently ignores this.

I think we should throw. This would let us add support for objects (e.g. for passive listener configuration) if we want to in a minor rather than a major.
{ReactDOM.unstable_createPortal(['Bye', 'Hi'], portalContainer)}
        </div>,
        container,
      );
      expect(portalContainer.innerHTML).toBe('ByeHi');
      expect(container.innerHTML).toBe('<div></div>');
      ReactDOM.render(
        <div>
          {ReactDOM.unstable_createPortal(null, portalContainer)}
        </div>,
        container,
      );
      expect(portalContainer.innerHTML).toBe('');
      expect(container.innerHTML).toBe('<div></div>');
    });
    it('should keep track of namespace across portals (simple)', () => {
      assertNamespacesMatch(
        <svg {...expectSVG}>
          <image {...expectSVG} />
          {usePortal(<div {...expectHTML} />)}
          <image {...expectSVG} />
        </svg>,
      );
      assertNamespacesMatch(
        <math {...expectMath}>
          <mi {...expectMath} />
          {usePortal(<div {...expectHTML} />)}
          <mi {...expectMath} />
        </math>,
      );