code: function (onNext, onError) {
          var self = this, subscriber = new Observable();
          subscriber._onNext = onNext;
          subscriber._onError = onError;
          subscriber._nextErr = genNextErr(subscriber.next);
  
          subscriber.publisher = self;
          self.subscribers.push(subscriber);
  
          return subscriber;
      }
docs: It will create a new Observable, like promise.
  @param  {Function} onNext
  @param  {Function} onError
  @return {Observable}