anchor.diff:  /// This is a ListView of text lines, with a monospace font and a border.
   class _ConsoleOutput extends StatefulWidget {
     const _ConsoleOutput({
  -    Key key,
  -    @required this.lines,
  +    Key? key,
  +    required this.lines,
       this.footer,
     }) : super(key: key);
   
     final ValueListenable<List<ConsoleLine>> lines;
   
  -  final Widget footer;
  +  final Widget? footer;
   
     @override
     _ConsoleOutputState createState() => _ConsoleOutputState();
  
positive.code:   }
  
    void _onConsoleLinesChanged() {
      final nextLines = widget.lines.value ?? const [];
      if (nextLines == _currentLines) return;
  
      var forceScrollIntoView = false;
  
negative.code: /// top-right corner.
  class Console extends StatelessWidget {
    const Console({
      this.controls = const <Widget>[],
      required this.lines,
      this.title,
      this.footer,
    }) : super();
  
    final Widget? title;
    final Widget? footer;
    final List<Widget> controls;
    final ValueListenable<List<ConsoleLine>> lines;
  
  