anchor.diff:    final Widget child;
   
     /// Disposable object to be disposed when the group is closed.
  -  final Disposable disposable;
  +  final Disposable? disposable;
   
     @override
     _HoverCardTooltipState createState() => _HoverCardTooltipState();
  
positive.code:   }
  
    HoverCard.fromHoverEvent({
      @required BuildContext context,
      @required PointerHoverEvent event,
      @required Widget contents,
      @required double width,
      String title,
    }) : this(
            context: context,
            contents: contents,
  
negative.code: /// A hover card based tooltip.
  class HoverCardTooltip extends StatefulWidget {
    const HoverCardTooltip({
      required this.enabled,
      required this.onHover,
      required this.child,
      this.disposable,
    });
  
  