feat(Subject): add rxSubscriber symbol

spec/subject-spec.js

@@ -19,6 +19,11 @@ describe('Subject', function () {
     subject.complete();
   });
 
+  it('should have the rxSubscriber Symbol', function () {
+    var subject = new Subject();
+    expect(subject[Rx.Symbol.rxSubscriber]()).toBe(subject);
+  });
+
   it('should pump values to multiple subscribers', function (done) {
     var subject = new Subject();
     var expected = ['foo', 'bar'];

src/Subject.ts

@@ -4,6 +4,7 @@ import {Observable} from './Observable';
 import {Subscriber} from './Subscriber';
 import {Subscription} from './Subscription';
 import {SubjectSubscription} from './subject/SubjectSubscription';
+import {rxSubscriber} from './symbol/rxSubscriber';
 
 const subscriptionAdd = Subscription.prototype.add;
 const subscriptionRemove = Subscription.prototype.remove;
@@ -20,6 +21,10 @@ export class Subject<T> extends Observable<T> implements Observer<T>, Subscripti
   _subscriptions: Subscription<T>[];
   _unsubscribe: () => void;
 
+  [rxSubscriber]() {
+    return this;
+  }
+
   static create<T>(source: Observable<T>, destination: Observer<T>): Subject<T> {
     return new BidirectionalSubject(source, destination);
   }