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.
container,
      );
      expect(portalContainer.innerHTML).toBe('<div>portal:1</div>');
      expect(container.innerHTML).toBe('<div></div>');
      ReactDOM.render(
        <div>
          {ReactDOM.unstable_createPortal(<div>portal:2</div>, portalContainer)}
        </div>,
        container,
      );
      expect(portalContainer.innerHTML).toBe('<div>portal:2</div>');
      expect(container.innerHTML).toBe('<div></div>');
      ReactDOM.render(
        <div>
          {ReactDOM.unstable_createPortal(<p>portal:3</p>, portalContainer)}
        </div>,
        container,
      );
      expect(portalContainer.innerHTML).toBe('<p>portal:3</p>');
      expect(container.innerHTML).toBe('<div></div>');
      ReactDOM.render(
        <div>
          {ReactDOM.unstable_createPortal(['Hi', 'Bye'], portalContainer)}
        </div>,
        container,
      );
      expect(portalContainer.innerHTML).toBe('HiBye');
      expect(container.innerHTML).toBe('<div></div>');
      ReactDOM.render(
        <div>