anchor.diff:    @override
     final String cssClass;
   
  -  IconAffix(
  +  const IconAffix(
         {@required this.icon,
         this.visibility = IconVisibility.visible,
         this.cssClass});
  
positive.code: 
  /// Represents a menu item content affix.
  abstract class MenuItemAffix {
    IconVisibility get visibility;
  
    bool get isVisibleOnHover => visibility == IconVisibility.hover;
  
negative.code:   @override
    final String cssClass;
  
    const CaptionAffix(
        {this.text, this.visibility = IconVisibility.visible, this.cssClass});
  
    @override
  