function AnonymousObserver(onNext, onError, onCompleted) {
      __super__.call(this);
      this._onNext = onNext;
      this._onError = onError;
      this._onCompleted = onCompleted;
    }
Creates an observer from the specified OnNext, OnError, and OnCompleted actions.
@param {Any} onNext Observer's OnNext action implementation.
@param {Any} onError Observer's OnError action implementation.
@param {Any} onCompleted Observer's OnCompleted action implementation.