anchor.diff:        _OptionValueWithContext<Object> valueWithContext, String missingPath) {
       var description = 'Synthetic configuration option ${name} from <internal>';
       throw DartdocFileMissing(
  -        '$description, computed as ${valueWithContext.value}, resolves to missing path: "${missingPath}"');
  +        '$description, computed as ${valueWithContext.value}, resolves to '
  +        'missing path: "${missingPath}"');
     }
   }
   
   typedef OptionGenerator = Future<List<DartdocOption<Object>>> Function();
   
  -/// A [DartdocOption] that only contains other [DartdocOption]s and is not an option itself.
  +/// A [DartdocOption] that only contains other [DartdocOption]s and is not an
  +/// option itself.
   class DartdocOptionSet extends DartdocOption<Null> {
     DartdocOptionSet(String name)
         : super(name, null, null, false, false, false, null);
  
positive.code:   void _onMissing(_OptionValueWithContext<Object> valueWithContext,
        String missingFilename) {}
  
    /// Traverse skips this node, because it doesn't represent a real configuration object.
    @override
    void traverse(void Function(DartdocOption<Object> option) visitor) {
      _children.values.forEach((d) => d.traverse(visitor));
    }
  }
  
  /// A [DartdocOption] that only exists as a command line argument. --help would
  /// be a good example.
  class DartdocOptionArgOnly<T> extends DartdocOption<T>
      with _DartdocArgOption<T> {
    String _abbr;
  
negative.code:     return value;
    }
  
    /// Searches all dartdoc_options files for the option, and returns the value
    /// in the top-most parent directory `dartdoc_options.yaml` file it is
    /// mentioned in.
    _OptionValueWithContext<Object> _valueAtFromFilesLastFound(Directory dir) {
      _OptionValueWithContext<Object> value;
  