anchor.diff:  
       // Check code contains only valid characters.
       for (int ch in code.codeUnits) {
  -      if (ch > decode_.length || decode_[ch] < -1) {
  +      if (ch > _decode.length || _decode[ch] < -1) {
           return false;
         }
       }
  
positive.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;
  
negative.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,  //
  