anchor.diff:  
     /// The background image of the circle. Changing the background
     /// image will cause the avatar to animate to the new image.
  +  ///
  +  /// If the [CircleAvatar] is to have the user's initials, use [child] instead.
     final ImageProvider backgroundImage;
   
     /// The size of the avatar. Changing the radius will cause the
     /// avatar to animate to the new size.
  +  ///
  +  /// Defaults to 20 logical pixels.
     final double radius;
   
     @override
     Widget build(BuildContext context) {
       final ThemeData theme = Theme.of(context);
       final Color color = backgroundColor ?? theme.primaryColor;
  -
       return new AnimatedContainer(
         width: radius * 2.0,
         height: radius * 2.0,
  
positive.code: 
  /// A circle that represents a user.
  ///
  /// Typicially used with a user's profile image, or, in the absence of
  /// such an image, the user's initials. A given user's initials should
  /// always be paired with the same background color, for consistency.
  ///
  /// See also:
  ///
  ///  * [Chip]
  ///  * [ListItem]
  ///  * <https://material.google.com/components/chips.html#chips-contact-chips>
  class CircleAvatar extends StatelessWidget {
    /// Creates a circle that represents a user.
  
negative.code:         backgroundImage: backgroundImage != null ? new BackgroundImage(
            image: backgroundImage
          ) : null,
          shape: BoxShape.circle,
        ),
        child: child != null ? new Center(
          child: new DefaultTextStyle(
            style: theme.primaryTextTheme.title,
            child: child,
          )
        ) : null,
      );
    }
  }
  