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.
ref={n => (secondTarget = n)}>
                portal
              </div>,
              portalContainer,
            )}
          </div>
          <div ref={n => (thirdTarget = n)} />
        </div>,
        container,
      );
      simulateMouseMove(null, firstTarget);
      expect(ops).toEqual(['enter parent']);
      ops = [];
      simulateMouseMove(firstTarget, secondTarget);
      expect(ops).toEqual([
        
        'enter portal',
      ]);
      ops = [];
      simulateMouseMove(secondTarget, thirdTarget);
      expect(ops).toEqual([
        'leave portal',
        'leave parent', 
      ]);
    });
    it('should not update event handlers until commit', () => {
      let ops = [];
      const handlerA = () => ops.push('A');
      const handlerB = () => ops.push('B');
      class Example extends React.Component {