function ScheduledObserver(scheduler, observer) {
      __super__.call(this);
      this.scheduler = scheduler;
      this.observer = observer;
      this.isAcquired = false;
      this.hasFaulted = false;
      this.queue = [];
      this.disposable = new SerialDisposable();
    }
Subscribes to the next value in the sequence with an optional "this" argument.
@param {Function} onCompleted The function to invoke upon graceful termination of the observable sequence.
@param {Any} [thisArg] Object to use as this when executing callback.
@returns {Disposable} A disposable handling the subscriptions and unsubscriptions.