feat(Subscriber): add rxSubscriber symbol

spec/subscriber-spec.js

@@ -0,0 +1,10 @@
+/* globals describe, it, expect */
+var Rx = require('../dist/cjs/Rx');
+var Subscriber = Rx.Subscriber;
+
+describe('Subscriber', function () {
+  it('should have the rxSubscriber symbol', function () {
+    var sub = new Subscriber();
+    expect(sub[Rx.Symbol.rxSubscriber]()).toBe(sub);
+  });
+});

src/Subscriber.ts

@@ -4,11 +4,16 @@ import {tryOrOnError} from './util/tryOrOnError';
 
 import {Observer} from './Observer';
 import {Subscription} from './Subscription';
+import {rxSubscriber} from './symbol/rxSubscriber';
 
 export class Subscriber<T> extends Subscription<T> implements Observer<T> {
   protected _subscription: Subscription<T>;
   protected _isUnsubscribed: boolean = false;
 
+  [rxSubscriber]() {
+    return this;
+  }
+
   get isUnsubscribed(): boolean {
     const subscription = this._subscription;
     if (subscription) {