refactor(skipUntil): call super(destination) constructor

Change SkipUntilSubscriber to give the destination subscriber to the
constructor in Subscriber. Adapt SkipUntilSubscriber to handle
unsubscription-on-complete correctly, by overriding unsubscribe().

Addresses ReactiveX#577.

src/Subscriber.ts

@@ -9,10 +9,10 @@ import Subscription from './Subscription';
 export default class Subscriber<T> extends Subscription<T> implements Observer<T> {
   protected destination: Observer<any>;
 
-  private _subscription: Subscription<T>;
+  protected _subscription: Subscription<T>;
+
+  protected _isUnsubscribed: boolean = false;
 
-  private _isUnsubscribed: boolean = false;
-
   static create<T>(next    ?: (x?:any) => void,
                    error   ?: (e?:any) => void,
                    complete?: () => void): Subscriber<T> {
@@ -22,23 +22,23 @@ export default class Subscriber<T> extends Subscription<T> implements Observer<T
     subscriber._complete = (typeof complete === "function") && complete || noop;
     return subscriber;
   }
-  
+
   _next(value: T) {
     this.destination.next(value);
   }
-  
+
   _error(err: any) {
     this.destination.error(err);
   }
-  
+
   _complete() {
     this.destination.complete();
   }
 
   constructor(destination?: Observer<any>) {
     super();
     this.destination = destination;
-    
+
     if (!destination) {
       return;
     }
@@ -90,9 +90,9 @@ export default class Subscriber<T> extends Subscription<T> implements Observer<T
   }
 
   unsubscribe() {
-    if(this._isUnsubscribed) {
+    if (this._isUnsubscribed) {
       return;
-    } else if(this._subscription) {
+    } else if (this._subscription) {
       this._isUnsubscribed = true;
     } else {
       super.unsubscribe();

src/operators/skipUntil.ts

@@ -19,9 +19,9 @@ class SkipUntilOperator<T, R> implements Operator<T, R> {
 class SkipUntilSubscriber<T> extends Subscriber<T> {
   private notificationSubscriber: NotificationSubscriber<any> = null;
 
-  constructor(public destination: Subscriber<T>,
+  constructor(destination: Subscriber<T>,
               private notifier: Observable<any>) {
-    super(null);
+    super(destination);
     this.notificationSubscriber = new NotificationSubscriber(this);
     this.add(this.notifier.subscribe(this.notificationSubscriber));
   }
@@ -42,6 +42,17 @@ class SkipUntilSubscriber<T> extends Subscriber<T> {
     }
     this.notificationSubscriber.unsubscribe();
   }
+
+  unsubscribe() {
+    if (this._isUnsubscribed) {
+      return;
+    } else if (this._subscription) {
+      this._subscription.unsubscribe();
+      this._isUnsubscribed = true;
+    } else {
+      super.unsubscribe();
+    }
+  }
 }
 
 class NotificationSubscriber<T> extends Subscriber<T> {