function DistinctUntilChangedObservable(source, keyFn, comparer) {
      this.source = source;
      this.keyFn = keyFn;
      this.comparer = comparer;
      __super__.call(this);
    }
Dematerializes the explicit notification values of an observable sequence as implicit notifications.
@returns {Observable} An observable sequence exhibiting the behavior corresponding to the source sequence's notification values.