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.
*     default, the listener is simply invoked.
 *
 * Each plugin that is injected into `EventsPluginHub` is immediately operable.
 *
 * @public
 */
var EventPluginHub = {
  /**
   * Methods for injecting dependencies.
   */
  injection: {
    /**
     * @param {array} InjectedEventPluginOrder
     * @public
     */
    injectEventPluginOrder: EventPluginRegistry.injectEventPluginOrder,
    /**
     * @param {object} injectedNamesToPlugins Map from names to plugin modules.
     */
    injectEventPluginsByName: EventPluginRegistry.injectEventPluginsByName,
  },
  /**
   * @param {object} inst The instance, which is the source of events.
   * @param {string} registrationName Name of listener (e.g. `onClick`).
   * @return {?function} The stored callback.
   */
  getListener: function(inst, registrationName) {
    var listener;