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 (
        shouldPreventMouseEvent(registrationName, currentElement.type, props)
      ) {
        return null;
      }
    }
    invariant(
      !listener || typeof listener === 'function',
      'Expected %s listener to be a function, instead got type %s',
      registrationName,
      typeof listener,
    );
    return listener;
  },
  /**
   * Allows registered plugins an opportunity to extract events from top-level
   * native browser events.
   *
   * @return {*} An accumulation of synthetic events.
   * @internal
   */
  extractEvents: function(
    topLevelType,
    targetInst,
    nativeEvent,
    nativeEventTarget,
  ) {
    var events;
    var plugins = EventPluginRegistry.plugins;
    for (var i = 0; i < plugins.length; i++) {