anchor.diff:      Key key,
       @required Animation<RelativeRect> rect,
       @required this.child,
  -  }) : super(key: key, listenable: rect);
  +  }) : assert(rect != null),
  +       super(key: key, listenable: rect);
   
     /// The animation that controls the child's size and position.
     Animation<RelativeRect> get rect => listenable;
  
positive.code:     @required this.decoration,
      this.position = DecorationPosition.background,
      @required this.child,
    }) : super(key: key, listenable: decoration);
  
    /// Animation of the decoration to paint.
    ///
  
negative.code:     @required Animation<Rect> rect,
      @required this.size,
      @required this.child,
    }) : assert(rect != null),
         assert(size != null),
         assert(child != null),
         super(key: key, listenable: rect);
  
    /// The animation that controls the child's size and position.
    ///
  