function FromObservable(iterable, fn, scheduler) {
      this._iterable = iterable;
      this._fn = fn;
      this._scheduler = scheduler;
      __super__.call(this);
    }
Returns an empty observable sequence, using the specified scheduler to send out the single OnCompleted message.

@example
 var res = Rx.Observable.empty();
 var res = Rx.Observable.empty(Rx.Scheduler.timeout);
@param {Scheduler} [scheduler] Scheduler to send the termination call on.
@returns {Observable} An observable sequence with no elements.