anchor.diff:    final List<Filter> _filters;
     final ClassMirror _instanceType;
     final bool _isList;
  -  final bool _isNullable;
  +  final bool _isOptional;
   
     _FinderFieldInfo(
         this._fieldName,
  
positive.code:     List elements = _getElements(context);
  
      if (!_isList) {
        if (elements.isEmpty && !_isNullable) {
          throw new StateError('Unable to find element for non-nullable, non-list'
              ' field $_fieldName');
        }
  
negative.code:     }
  
      if (finder != null) {
        return new _FinderFieldInfo(name, finder, filters, type, isList, isOptional);
      } else if (type.simpleName == const Symbol('PageLoader')) {
        return new _InjectedPageLoaderInfo(name);
      } else if (type.simpleName == const Symbol('WebDriver')) {
  