EventEmitter: Deprecate Constructor Argument

Summary:
Deprecates the constructor argument to `EventEmitter`.

It was only ever used by React Native to instantiate a `NativeEventEmitter` with the same "vendor" as `RCTDeviceEventEmitter`, which was a gross violation of encapsulation that led to an overcomplicated implementation of `EventEmitter`. I am now untangling that mess.

Changelog:
[General][Deprecated] - It is now deprecated to pass a constructor argument to `EventEmitter(...)`.

Reviewed By: rubennorte

Differential Revision: D27704182

fbshipit-source-id: 102f26a8a9c029f0e7a1346dc2f2d9f2548d6a42

Libraries/vendor/emitter/_EventEmitter.js

@@ -46,17 +46,16 @@ export interface IEventEmitter<EventDefinitions: {...}> {
  */
 class EventEmitter<EventDefinitions: {...}>
   implements IEventEmitter<EventDefinitions> {
-  _subscriber: EventSubscriptionVendor<EventDefinitions>;
+  _subscriber: EventSubscriptionVendor<EventDefinitions> = new EventSubscriptionVendor<EventDefinitions>();
 
   /**
    * @constructor
-   *
-   * @param {EventSubscriptionVendor} subscriber - Optional subscriber instance
-   *   to use. If omitted, a new subscriber will be created for the emitter.
    */
   constructor(subscriber: ?EventSubscriptionVendor<EventDefinitions>) {
-    this._subscriber =
-      subscriber || new EventSubscriptionVendor<EventDefinitions>();
+    if (subscriber != null) {
+      console.warn('EventEmitter(...): Constructor argument is deprecated.');
+      this._subscriber = subscriber;
+    }
   }
 
   /**