function EmptyObservable(scheduler) {
      this.scheduler = scheduler;
      __super__.call(this);
    }
Returns an observable sequence that invokes the specified factory function whenever a new observer subscribes.

@example
 var res = Rx.Observable.defer(function () { return Rx.Observable.fromArray([1,2,3]); });
@param {Function} observableFactory Observable factory function to invoke for each observer that subscribes to the resulting sequence or Promise.
@returns {Observable} An observable sequence whose observers trigger an invocation of the given observable factory function.