EventEmitter: Deprecate removeSubscription

Summary:
Deprecates `EventEmitter#removeSubscription`. This required temporarily introducing a new `__removeSubscription` method that is only invoked by `EmitterSubscription`. This is necessary so that we do not completely break usages of `EventEmitter` that are supplying constructor arguments (which is also deprecated, but still supported until the next release).

Calling this method will now cause a warning to appear with instructions to instead invoke `remove()` on the subscription itself.

Lastly, changed `console.error` deprecation notice to instead use `console.warn`. This is in line with the principle that errors are "broken today" and warnings will be "broken tomorrow".

Changelog:
[General][Deprecated] - `EventEmitter#removeSubscription` is now deprecated.

Reviewed By: rubennorte

Differential Revision: D27704279

fbshipit-source-id: 581f5b2ab46b1bcfc1d20898b3d3392988dccbd5

Author: yungsters

Libraries/vendor/emitter/_EmitterSubscription.js

@@ -54,7 +54,7 @@ class EmitterSubscription<EventDefinitions: {...}, K: $Keys<EventDefinitions>>
    * for removing the subscription lies with the EventEmitter.
    */
   remove(): void {
-    this.emitter.removeSubscription(this);
+    this.emitter.__removeSubscription(this);
   }
 }
 

Libraries/vendor/emitter/_EventEmitter.js

@@ -82,6 +82,19 @@ class EventEmitter<EventDefinitions: {...}> {
    */
   removeSubscription<K: $Keys<EventDefinitions>>(
     subscription: EmitterSubscription<EventDefinitions, K>,
+  ): void {
+    console.warn(
+      'EventEmitter.removeSubscription(...): Method has been deprecated. ' +
+        'Please instead use `remove()` on the subscription itself.',
+    );
+    this.__removeSubscription(subscription);
+  }
+
+  /**
+   * Called by `EmitterSubscription` to bypass the above deprecation warning.
+   */
+  __removeSubscription<K: $Keys<EventDefinitions>>(
+    subscription: EmitterSubscription<EventDefinitions, K>,
   ): void {
     invariant(
       subscription.emitter === this,
@@ -147,7 +160,7 @@ class EventEmitter<EventDefinitions: {...}> {
     // FIXME: listeners should return void instead of mixed to prevent issues
     listener: (...$ElementType<EventDefinitions, K>) => mixed,
   ): void {
-    console.error(
+    console.warn(
       `EventEmitter.removeListener('${eventType}', ...): Method has been ` +
         'deprecated. Please instead use `remove()` on the subscription ' +
         'returned by `EventEmitter.addListener`.',