anchor.diff:      List elements = _getElements(context);
   
       if (!_isList) {
  -      if (elements.isEmpty && !_isNullable) {
  +      if (elements.isEmpty && !_isOptional) {
           throw new StateError('Unable to find element for non-nullable, non-list'
               ' field $_fieldName');
         }
  
positive.code:       type = null;
      }
  
      var isNullable = false;
  
      var implicitDisplayFiltering = true;
  
  
negative.code:       this._filters,
        this._instanceType,
        this._isList,
        this._isOptional);
  
    @override
    void setField(
  