function SliceObservable(source, b, e) {
      this.source = source;
      this._b = b;
      this._e = e;
      __super__.call(this);
    }
Converts the observable sequence to a Map if it exists.
@param {Function} keySelector A function which produces the key for the Map.
@param {Function} [elementSelector] An optional function which produces the element for the Map. If not present, defaults to the value from the observable sequence.
@returns {Observable} An observable sequence with a single value of a Map containing the values from the observable sequence.