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