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.
'<div>portal2[0]:a</div><div>portal2[1]:a</div>',
      );
      expect(container.innerHTML).toBe(
        '<div>normal[0]:a</div><div>normal[1]:a</div>',
      );
      expect(ops).toEqual([
        'normal[0]:a componentDidMount',
        'portal1[0]:a componentDidMount',
        'normal[1]:a componentDidMount',
        'portal2[0]:a componentDidMount',
        'portal2[1]:a componentDidMount',
        'Parent:a componentDidMount',
      ]);
      ops.length = 0;
      ReactDOM.render(<Parent step="b" />, container);
      expect(portalContainer1.innerHTML).toBe('<div>portal1[0]:b</div>');
      expect(portalContainer2.innerHTML).toBe(
        '<div>portal2[0]:b</div><div>portal2[1]:b</div>',
      );
      expect(container.innerHTML).toBe(
        '<div>normal[0]:b</div><div>normal[1]:b</div>',
      );
      expect(ops).toEqual([
        'normal[0]:b componentDidUpdate',
        'portal1[0]:b componentDidUpdate',
        'normal[1]:b componentDidUpdate',
        'portal2[0]:b componentDidUpdate',
        'portal2[1]:b componentDidUpdate',
        'Parent:b componentDidUpdate',
      ]);