anchor.diff:        return;
       }
   
  -    _addChild(new CssDeclaration(name, cssValue, span: node.span));
  +    _parent.addChild(new CssDeclaration(name, cssValue, span: node.span));
     }
   
     void visitAtRule(AtRule node) {
  
positive.code:   CssValue<Value> _performExpression(Expression expression) =>
        new CssValue(expression.accept(_expressionVisitor));
  
    void _addChild(CssNode node) {
      _parent.addChild(node);
    }
  
    /*=T*/ _withParent/*<S extends CssParentNode, T>*/(
        /*=S*/ node, /*=T*/ callback(),
        {bool through(CssNode node), bool removeIfEmpty: false}) {
  
negative.code: 
    void visitComment(Comment node) {
      if (node.isSilent) return;
      _parent.addChild(new CssComment(node.text, span: node.span));
    }
  
    void visitDeclaration(Declaration node) {
  