anchor.diff:    final bool _isOptional;
   
     _FinderSingleFieldInfo(Symbol fieldName, this._finder, this._filters,
  -                         this._instanceType, this._isOptional)
  -  : super(fieldName);
  +      this._instanceType, this._isOptional)
  +      : super(fieldName);
   
     @override
     calculateFieldValue(SearchContext context, PageLoader loader) {
       var element = _getElement(context, _finder, _filters, _isOptional);
       if (_instanceType.simpleName != const Symbol('WebElement') &&
  -    element != null) {
  +        element != null) {
         element = loader._getInstance(_instanceType, element);
       }
       return element;
  
positive.code:     try {
        if (_finder != null) {
          SearchContext ctx =
          _getElement(context, _finder, _filters, _finderIsOptional);
          if (ctx != null) {
            context = ctx;
          }
  
negative.code:   });
  
    _ClassInfo._(this._class, this._fields, this._finder, this._filters,
        this._finderIsOptional);
  
    static Iterable<_FieldInfo> _fieldInfos(ClassMirror type) {
      var infos = <_FieldInfo>[];
  