anchor.diff:      @required this.child,
       this.widthFactor,
       this.heightFactor,
  -  }) : super(key: key, listenable: alignment);
  +  }) : assert(alignment != null),
  +       assert(child != null),
  +       super(key: key, listenable: alignment);
   
     /// The animation that controls the child's alignment.
     Animation<AlignmentGeometry> get alignment => listenable;
  
positive.code:     Key key,
      @required Animation<RelativeRect> rect,
      @required this.child,
    }) : super(key: key, listenable: rect);
  
    /// The animation that controls the child's size and position.
    Animation<RelativeRect> get rect => listenable;
  
negative.code:     this.softWrap = true,
      this.overflow = TextOverflow.clip,
      this.maxLines,
    }) : assert(style != null),
         assert(child != null),
         super(key: key, listenable: style);
  
    /// The animation that controls the descendants' text style.
    Animation<TextStyle> get style => listenable;
  