EventEmitter: Cleanup Redundant Internal Types

Summary:
Cleans up some minor redundant internal types in `_EventEmitter.js`.

Changelog:
[Internal]

Reviewed By: rubennorte

Differential Revision: D27712301

fbshipit-source-id: e6affcb7948e0f47a1775b74b12f59ec2a6abddb

Libraries/vendor/emitter/_EventEmitter.js

@@ -12,25 +12,11 @@
 const invariant = require('invariant');
 
 import EmitterSubscription from './_EmitterSubscription';
+import {type EventSubscription} from './EventSubscription';
 import EventSubscriptionVendor from './_EventSubscriptionVendor';
 
 const sparseFilterPredicate = () => true;
 
-export interface IEventEmitter<EventDefinitions: {...}> {
-  addListener<K: $Keys<EventDefinitions>>(
-    eventType: K,
-    listener: (...$ElementType<EventDefinitions, K>) => mixed,
-    context: $FlowFixMe,
-  ): EmitterSubscription<EventDefinitions, K>;
-
-  removeAllListeners<K: $Keys<EventDefinitions>>(eventType: ?K): void;
-
-  emit<K: $Keys<EventDefinitions>>(
-    eventType: K,
-    ...args: $ElementType<EventDefinitions, K>
-  ): void;
-}
-
 /**
  * @class EventEmitter
  * @description
@@ -44,8 +30,7 @@ export interface IEventEmitter<EventDefinitions: {...}> {
  * mechanism on top of which extra functionality can be composed. For example, a
  * more advanced emitter may use an EventHolder and EventFactory.
  */
-class EventEmitter<EventDefinitions: {...}>
-  implements IEventEmitter<EventDefinitions> {
+class EventEmitter<EventDefinitions: {...}> {
   _subscriber: EventSubscriptionVendor<EventDefinitions> = new EventSubscriptionVendor<EventDefinitions>();
 
   /**
@@ -63,9 +48,6 @@ class EventEmitter<EventDefinitions: {...}>
    * emitted. An optional calling context may be provided. The data arguments
    * emitted will be passed to the listener function.
    *
-   * TODO: Annotate the listener arg's type. This is tricky because listeners
-   *       can be invoked with varargs.
-   *
    * @param {string} eventType - Name of the event to listen to
    * @param {function} listener - Function to invoke when the specified event is
    *   emitted
@@ -77,7 +59,7 @@ class EventEmitter<EventDefinitions: {...}>
     // FIXME: listeners should return void instead of mixed to prevent issues
     listener: (...$ElementType<EventDefinitions, K>) => mixed,
     context: $FlowFixMe,
-  ): EmitterSubscription<EventDefinitions, K> {
+  ): EventSubscription {
     return (this._subscriber.addSubscription(
       eventType,
       new EmitterSubscription(this, this._subscriber, listener, context),