anchor.diff:    Widget build(BuildContext context) {
       Color backgroundColor = _getBackgroundColor(context);
       Widget contents = config.child;
  +    BorderRadius radius = config.borderRadius ?? kMaterialEdges[config.type];
       if (contents != null) {
         contents = new AnimatedDefaultTextStyle(
           style: config.textStyle ?? Theme.of(context).textTheme.body1,
  
positive.code:       contents = new ClipOval(child: contents);
      } else if (kMaterialEdges[config.type] != null) {
        contents = new ClipRRect(
          borderRadius: kMaterialEdges[config.type],
          child: contents
        );
      }
  
negative.code: 
    /// The typographical style to use for text within this material.
    final TextStyle textStyle;
  
    /// If non-null, the corners of this box are rounded by this [BorderRadius].
    /// Otherwise, the corners specified for the current [type] of material are
    /// used.
    ///
    /// Must be null if [type] is [MaterialType.circle].
    final BorderRadius borderRadius;
  
    /// The ink controller from the closest instance of this class that
    /// encloses the given context.
  