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.
if (from) {
          ReactTestUtils.simulateNativeEventOnNode('topMouseOut', from, {
            target: from,
            relatedTarget: to,
          });
        }
        if (to) {
          ReactTestUtils.simulateNativeEventOnNode('topMouseOver', to, {
            target: to,
            relatedTarget: from,
          });
        }
      }
      ReactDOM.render(
        <div>
          <div
            onMouseEnter={() => ops.push('enter parent')}
            onMouseLeave={() => ops.push('leave parent')}>
            <div ref={n => (firstTarget = n)} />
            {ReactDOM.unstable_createPortal(
              <div
                onMouseEnter={() => ops.push('enter portal')}
                onMouseLeave={() => ops.push('leave portal')}