anchor.diff:     * Get a [Converter] for encoding to `ZLib` compressed data.
      */
     Converter<List<int>, List<int>> get encoder =>
  -      const ZLibEncoder(gzip: false, level: level);
  +      new ZLibEncoder(gzip: false, level: level);
   
     /**
      * Get a [Converter] for decoding `ZLib` compressed data.
  
positive.code:   /**
     * Get a [Converter] for encoding to `GZip` compressed data.
     */
    final Converter<List<int>, List<int>> encoder =
        const ZLibEncoder(gzip: true, level: level);
  
    /**
     * Get a [Converter] for decoding `GZip` compressed data.
     */
    final Converter<List<int>, List<int>> decoder = const ZLibDecoder();
  
    /**
     * The compression-[level] can be set in the range of `1..10`, with `6` being
  
negative.code:   void addSlice(List<int> data, int start, int end, bool isLast) {
      if (_closed) return;
      if (start < 0 || start > data.length) {
        throw new ArgumentError("Invalid start position");
      }
      if (end < 0 || end > data.length || end < start) {
        throw new ArgumentError("Invalid end position");
      }
      try {
        _empty = false;
  