anchor.diff:    }
   
     void _writeGraphicsCtrlExt() {
  -    output!.writeByte(EXTENSION_RECORD_TYPE);
  -    output!.writeByte(GRAPHIC_CONTROL_EXT);
  +    output!.writeByte(_extensionRecordType);
  +    output!.writeByte(_graphicControlExt);
       output!.writeByte(4); // data block size
   
       const transparency = 0;
  
positive.code: 
    // GIF header and Logical Screen Descriptor
    void _writeHeader(int width, int height) {
      output!.writeBytes(GIF89_STAMP.codeUnits);
      output!.writeUint16(width);
      output!.writeUint16(height);
      output!.writeByte(0); // global color map parameters (not being used).
  
negative.code: 
      int _nextPixel() {
        if (remaining == 0) {
          return _eof;
        }
        --remaining;
        return image![curPixel++] & 0xff;
  