function TakeUntilObservable(source, other) {
      this.source = source;
      this.other = isPromise(other) ? observableFromPromise(other) : other;
      __super__.call(this);
    }
Transforms an observable sequence of observable sequences into an observable sequence producing values only from the most recent observable sequence.
@returns {Observable} The observable sequence that at any point in time produces the elements of the most recent inner observable sequence that has been received.