anchor.diff:      _navigator = null;
     }
   
  -  // The drag gesture has changed by [fractionalDelta]. The total range of the
  -  // drag should be 0.0 to 1.0.
  +  /// The drag gesture has changed by [fractionalDelta]. The total range of the
  +  /// drag should be 0.0 to 1.0.
     void dragUpdate(double fractionalDelta);
   
  -  // The drag gesture has ended with a horizontal motion of
  -  // [fractionalVelocity] as a fraction of screen width per second.
  +  /// The drag gesture has ended with a horizontal motion of
  +  /// [fractionalVelocity] as a fraction of screen width per second.
     void dragEnd(double fractionalVelocity);
   }
   
  
positive.code:       ..pushNamed(routeName);
    }
  
    static NavigatorState of(BuildContext context) {
      NavigatorState navigator = context.ancestorStateOfType(const TypeMatcher<NavigatorState>());
      assert(() {
  
negative.code:     return _history.length > 1 || _history[0].willHandlePopInternally;
    }
  
    /// Starts a gesture that results in popping the navigator.
    NavigationGestureController startPopGesture() {
      if (canPop())
        return _history.last.startPopGesture(this);
      return null;
    }
  
    /// Whether a gesture controlled by a [NavigationGestureController] is currently in progress.
    bool get userGestureInProgress => _userGestureInProgress;
    // TODO(mpcomplete): remove this bool when we fix
    // https://github.com/flutter/flutter/issues/5577
    bool _userGestureInProgress = false;
  
    /// The navigator is being controlled by a user gesture.
    ///
    /// Used for the iOS back gesture.
    void didStartUserGesture() {
      _userGestureInProgress = true;
      config.observer?.didStartUserGesture();
    }
  
    /// A user gesture is no longer controlling the navigator.
    void didStopUserGesture() {
      _userGestureInProgress = false;
      config.observer?.didStopUserGesture();
  