anchor.diff:  
   class ReleaseNotes extends AnimatedWidget {
     const ReleaseNotes({
  -    Key key,
  -    @required this.releaseNotesController,
  -    @required Animation<double> visibilityAnimation,
  -    @required this.markdownData,
  +    Key? key,
  +    required this.releaseNotesController,
  +    required Animation<double> visibilityAnimation,
  +    required this.markdownData,
     }) : super(key: key, listenable: visibilityAnimation);
   
     final ReleaseNotesController releaseNotesController;
   
  -  final String markdownData;
  +  final String? markdownData;
   
     @override
     Widget build(BuildContext context) {
  
positive.code: // @dart=2.9
  
  import 'dart:async';
  
  
negative.code:   static const viewerWidth = 600.0;
  
    /// Animation controller for animating the opening and closing of the viewer.
    late AnimationController visibilityController;
  
    /// A curved animation that matches [visibilityController].
    late Animation<double> visibilityAnimation;
  
    String? markdownData;
  
    late bool isVisible;
  
    @override
    void initState() {
  