anchor.diff:    }
   }
   
  +/// Displays a diagonal message above the corner of another widget.
  +///
  +/// Useful for showing the execution mode of an app (e.g., that asserts are
  +/// enabled.)
  +///
  +/// See also:
  +///
  +///  * [CheckedModeBanner]
   class Banner extends StatelessWidget {
  +  /// Creates a banner.
  +  ///
  +  /// The message and location arguments are required.
     Banner({
       Key key,
       this.child,
       this.message,
       this.location
  -  }) : super(key: key);
  +  }) : super(key: key) {
  +    assert(message != null);
  +    assert(location != null);
  +  }
   
  +  /// The widget to show behind the banner.
     final Widget child;
  +
  +  /// The message to show in the banner.
     final String message;
  +
  +  /// Where to show the banner (e.g., the upper right corder).
     final BannerLocation location;
   
     @override
  
positive.code:   }
  }
  
  /// Displays a banner saying "SLOW MODE" when running in checked mode.
  /// Does nothing in release mode.
  class CheckedModeBanner extends StatelessWidget {
    CheckedModeBanner({
      Key key,
      this.child
    }) : super(key: key);
  
    final Widget child;
  
    @override
  
negative.code:   double _translationX(double width) {
      switch (location) {
        case BannerLocation.bottomRight:
          return width - _kBottomOffset;
        case BannerLocation.topRight:
          return width;
        case BannerLocation.bottomLeft:
          return _kBottomOffset;
        case BannerLocation.topLeft:
          return 0.0;
      }
  