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.
var possiblePlugin = plugins[i];
      if (possiblePlugin) {
        var extractedEvents = possiblePlugin.extractEvents(
          topLevelType,
          targetInst,
          nativeEvent,
          nativeEventTarget,
        );
        if (extractedEvents) {
          events = accumulateInto(events, extractedEvents);
        }
      }
    }
    return events;
  },
  /**
   * Enqueues a synthetic event that should be dispatched when
   * `processEventQueue` is invoked.
   *
   * @param {*} events An accumulation of synthetic events.
   * @internal
   */
  enqueueEvents: function(events) {
    if (events) {
      eventQueue = accumulateInto(eventQueue, events);
    }
  },
  /**
   * Dispatches all synthetic events on the event queue.