function MapObservable(source, selector, thisArg) {
      this.source = source;
      this.selector = bindCallback(selector, thisArg, 3);
      __super__.call(this);
    }
Returns a specified number of contiguous elements from the end of an observable sequence.
@description
 This operator accumulates a buffer with a length enough to store elements count elements. Upon completion of
 the source sequence, this buffer is drained on the result sequence. This causes the elements to be delayed.
@param {Number} count Number of elements to take from the end of the source sequence.
@returns {Observable} An observable sequence containing the specified number of elements from the end of the source sequence.