anchor.diff:      @required this.opacity,
       this.alwaysIncludeSemantics = false,
       Widget child,
  -  }) : super(key: key, child: child);
  +  }) : assert(opacity != null),
  +       super(key: key, child: child);
   
     /// The animation that controls the opacity of the child.
     ///
  
positive.code:     @required Animation<double> scale,
      this.alignment = Alignment.center,
      this.child,
    }) : super(key: key, listenable: scale);
  
    /// The animation that controls the scale of the child.
    ///
  
negative.code:     @required this.child,
      this.widthFactor,
      this.heightFactor,
    }) : assert(alignment != null),
         assert(child != null),
         super(key: key, listenable: alignment);
  
    /// The animation that controls the child's alignment.
    Animation<AlignmentGeometry> get alignment => listenable;
  