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.
*
   * @internal
   */
  processEventQueue: function(simulated) {
    
    
    var processingEventQueue = eventQueue;
    eventQueue = null;
    if (simulated) {
      forEachAccumulated(
        processingEventQueue,
        executeDispatchesAndReleaseSimulated,
      );
    } else {
      forEachAccumulated(
        processingEventQueue,
        executeDispatchesAndReleaseTopLevel,
      );
    }
    invariant(
      !eventQueue,
      'processEventQueue(): Additional events were enqueued while processing ' +
        'an event queue. Support for this has not yet been implemented.',
    );
    
    ReactErrorUtils.rethrowCaughtError();
  },
};
module.exports = EventPluginHub;