anchor.diff:              'Use <base href> in generated files (legacy behavior). This option '
               'is temporary and support will be removed in the future. Use only '
               'if the default behavior breaks links between your documentation '
  -            'pages, and please file an issue on Github.',
  +            'pages, and please file an issue on GitHub.',
           negatable: false,
           hide: true),
       // TODO(jdkoren): Unhide when we have good support for another format.
  
positive.code:     return DartdocOptionContext(optionSet, File(element.source.fullName));
    }
  
    /// Build a DartdocOptionContext from an existing [DartdocOptionContext] and a new analyzer [Element].
    factory DartdocOptionContext.fromContextElement(
        DartdocOptionContext optionContext, Element element) {
      return DartdocOptionContext.fromElement(optionContext.optionSet, element);
  
negative.code:     return value;
    }
  
    /// Returns null if not set in the YAML file in this directory (or its
    /// parents).
    _OptionValueWithContext<Object> _valueAtFromFile(Directory dir) {
      var yamlFileData = _yamlAtDirectory(dir);
  