function FindValueObserver(observer, source, callback, yieldIndex) {
      this._o = observer;
      this._s = source;
      this._cb = callback;
      this._y = yieldIndex;
      this._i = 0;
      __super__.call(this);
    }
Returns the last element of an observable sequence that satisfies the condition in the predicate if specified, else the last element.
@returns {Observable} Sequence containing the last element in the observable sequence that satisfies the condition in the predicate.