function FirstObservable(source, obj) {
      this.source = source;
      this._obj = obj;
      __super__.call(this);
    }
Returns the only element of an observable sequence that satisfies the condition in the optional predicate, and reports an exception if there is not exactly one element in the observable sequence.
@returns {Observable} Sequence containing the single element in the observable sequence that satisfies the condition in the predicate.