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 (typeof inst.tag === 'number') {
      const stateNode = inst.stateNode;
      if (!stateNode) {
        
        return null;
      }
      const props = EventPluginUtils.getFiberCurrentPropsFromNode(stateNode);
      if (!props) {
        
        return null;
      }
      listener = props[registrationName];
      if (shouldPreventMouseEvent(registrationName, inst.type, props)) {
        return null;
      }
    } else {
      const currentElement = inst._currentElement;
      if (
        typeof currentElement === 'string' ||
        typeof currentElement === 'number'
      ) {
        
        return null;
      }
      if (!inst._rootNodeID) {
        
        return null;
      }
      const props = currentElement.props;
      listener = props[registrationName];