anchor.diff:  }
   
   class _MatchImplementation implements Match {
  +  final String pattern;
  +  final String str;
  +  final int _start;
  +  final int _end;
  +  final List<String> _groups;
  +
     const _MatchImplementation(
         String this.pattern,
         String this.str,
  
positive.code:       int this._end,
        List<String> this._groups);
  
    final String pattern;
    final String str;
    final int _start;
    final int _end;
    final List<String> _groups;
  
    int start() => _start;
    int end() => _end;
    String group(int index) => _groups[index];
  
negative.code:       return true;
      }
  
      // firstMatch actually acts as nextMatch because of
      // hidden global flag.
      _next = _re.firstMatch(_str);
      if (_next == null) {
        _done = true;
  