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.
ops.length = 0;
      ReactDOM.unmountComponentAtNode(container);
      expect(portalContainer1.innerHTML).toBe('');
      expect(portalContainer2.innerHTML).toBe('');
      expect(container.innerHTML).toBe('');
      expect(ops).toEqual([
        'Parent:b componentWillUnmount',
        'normal[0]:b componentWillUnmount',
        'portal1[0]:b componentWillUnmount',
        'normal[1]:b componentWillUnmount',
        'portal2[0]:b componentWillUnmount',
        'portal2[1]:b componentWillUnmount',
      ]);
    });
    it('should render nested portals', () => {
      var portalContainer1 = document.createElement('div');
      var portalContainer2 = document.createElement('div');
      var portalContainer3 = document.createElement('div');
      ReactDOM.render(
        [
          <div key="a">normal[0]</div>,
          ReactDOM.unstable_createPortal(
            [
              <div key="b">portal1[0]</div>,
              ReactDOM.unstable_createPortal(
                <div key="c">portal2[0]</div>,
                portalContainer2,
              ),
              ReactDOM.unstable_createPortal(
                <div key="d">portal3[0]</div>,