anchor.diff:      int count = 0;
       for (int i = _lowerIndex; i <= _upperIndex; i++) {
         final Color color = colors[i];
  -      count += histogram[color];
  +      count += histogram[color].value;
         if (color.red > maxRed) {
           maxRed = color.red;
         }
  
positive.code: 
    List<PaletteColor> _quantizePixels(
      int maxColors,
      Map<Color, int> histogram,
    ) {
      int volumeComparator(_ColorVolumeBox a, _ColorVolumeBox b) {
        return b.getVolume().compareTo(a.getVolume());
  
negative.code:     _fitMinimumBox();
    }
  
    final _ColorHistogram histogram;
    final List<Color> colors;
  
    // The lower and upper index are inclusive.
  