anchor.diff:      int i = 0;
       num value = 0;
       while (i * 2 + offset < code.length) {
  -      value += decode_[code.codeUnitAt(i * 2 + offset.floor())] * PAIR_RESOLUTIONS[i];
  +      value += _decode[code.codeUnitAt(i * 2 + offset.floor())] * PAIR_RESOLUTIONS[i];
         i += 1;
       }
       return [value, value + PAIR_RESOLUTIONS[i - 1]];
  
positive.code: /// * -2: illegal.
  /// * -1: Padding or Separator
  /// * >= 0: index in the alphabet.
  const decode_ = const <int>[
      -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  //
      -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  //
      -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -1, -2, -2, -2, -2,  //
  
negative.code:       return false;
      }
      // Work out what the first latitude character indicates for latitude.
      var firstLatValue = _decode[code.codeUnitAt(0)] * ENCODING_BASE;
      if (firstLatValue >= LATITUDE_MAX * 2) {
        // The code would decode to a latitude of >= 90 degrees.
        return false;
      }
      if (code.length > 1) {
        // Work out what the first longitude character indicates for longitude.
        var firstLngValue = _decode[code.codeUnitAt(1)] * ENCODING_BASE;
        if (firstLngValue >= LONGITUDE_MAX * 2) {
          // The code would decode to a longitude of >= 180 degrees.
          return false;
  