EventEmitter: Replace listeners() with listenerCount()

Summary:
Replaces the `listeners()` method on `EventEmitter` with a `listenerCount()` method.

Changelog:
[General][Removed] - Removed `listeners()` from `DeviceEventEmitter` and `NativeEventEmitter`.
[General][Added] - Added `listenerCount()` to `DeviceEventEmitter` and `NativeEventEmitter`.

Reviewed By: cpojer

Differential Revision: D22204156

fbshipit-source-id: 15029525aeef55de9934a4f319910e666ecbe1d8

Libraries/EventEmitter/NativeEventEmitter.js

@@ -50,7 +50,7 @@ export default class NativeEventEmitter extends EventEmitter {
 
   removeAllListeners(eventType: string) {
     invariant(eventType, 'eventType argument is required.');
-    const count = this.listeners(eventType).length;
+    const count = this.listenerCount(eventType);
     if (this._nativeModule != null) {
       this._nativeModule.removeListeners(count);
     }

Libraries/vendor/emitter/_EventEmitter.js

@@ -93,23 +93,21 @@ class EventEmitter {
   }
 
   /**
-   * Returns an array of listeners that are currently registered for the given
+   * Returns the number of listeners that are currently registered for the given
    * event.
    *
    * @param {string} eventType - Name of the event to query
-   * @returns {array}
+   * @returns {number}
    */
-  listeners(eventType: string): [EmitterSubscription] {
+  listenerCount(eventType: string): number {
     const subscriptions = this._subscriber.getSubscriptionsForType(eventType);
     return subscriptions
-      ? subscriptions
-          // We filter out missing entries because the array is sparse.
-          // "callbackfn is called only for elements of the array which actually
-          // exist; it is not called for missing elements of the array."
-          // https://www.ecma-international.org/ecma-262/9.0/index.html#sec-array.prototype.filter
-          .filter(sparseFilterPredicate)
-          .map(subscription => subscription.listener)
-      : [];
+      ? // We filter out missing entries because the array is sparse.
+        // "callbackfn is called only for elements of the array which actually
+        // exist; it is not called for missing elements of the array."
+        // https://www.ecma-international.org/ecma-262/9.0/index.html#sec-array.prototype.filter
+        subscriptions.filter(sparseFilterPredicate).length
+      : 0;
   }
 
   /**