anchor.diff:      if (name != null) {
         parts.add(name!);
       }
  -    parts.add(file);
  +    if (file != null) {
  +      parts.add(file!);
  +    }
       parts..add('$line')..add('$column');
       return parts.join(':');
     }
  
positive.code:       final Map<String, List<int>> locationsJson = <String, List<int>>{};
        for (final _LocationCount entry in newLocations) {
          final _Location location = entry.location;
          final List<int> jsonForFile = locationsJson.putIfAbsent(
            location.file,
            () => <int>[],
          );
          jsonForFile..add(entry.id)..add(location.line)..add(location.column);
        }
        json['newLocations'] = locationsJson;
      }
  
negative.code:   });
  
    /// File path of the location.
    final String? file;
  
    /// 1-based line number.
    final int line;
  