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.
var portal = null;
      ReactDOM.render(
        <div onClick={() => ops.push('parent clicked')}>
          {ReactDOM.unstable_createPortal(
            <div
              onClick={() => ops.push('portal clicked')}
              ref={n => (portal = n)}>
              portal
            </div>,
            portalContainer,
          )}
        </div>,
        container,
      );
      expect(portal.tagName).toBe('DIV');
      var fakeNativeEvent = {};
      ReactTestUtils.simulateNativeEventOnNode(
        'topClick',
        portal,
        fakeNativeEvent,
      );
      expect(ops).toEqual(['portal clicked', 'parent clicked']);
    });
    it('should not onMouseLeave when staying in the portal', () => {
      var portalContainer = document.createElement('div');
      var ops = [];
      var firstTarget = null;
      var secondTarget = null;
      var thirdTarget = null;
      function simulateMouseMove(from, to) {