anchor.diff:  
     final _unscopedSelectorGroups = <SelectorGroup>{};
   
  -  void shimPolyfillNextSelector(List<TreeNode> list) {
  +  void _shimPolyfillNextSelector(List<TreeNode> list) {
       SelectorGroup nextSelectorGroup;
   
       // Apply 'polyfill-next-selector' transformations.
  
positive.code:         : false);
    }
  
    void shimPolyfillUnscopedRule(RuleSet ruleSet) {
      if (_matchesElement(ruleSet.selectorGroup, 'polyfill-unscoped-rule')) {
        var contentSelectorGroup = _selectorGroupForProperty(
            ruleSet.declarationGroup, 'content',
  
negative.code:     super.shimSelectors(selector);
    }
  
    @override
    void visitTree(StyleSheet tree) {
      tree.visit(this);
      _unscopedSelectorGroups.clear();
    }
  
    @override
    void visitDeclarationGroup(DeclarationGroup node) {
      _shimPolyfillNextSelector(node.declarations);
      super.visitDeclarationGroup(node);
    }
  
    @override
    void visitMediaDirective(MediaDirective node) {
      _shimPolyfillNextSelector(node.rules);
      super.visitMediaDirective(node);
    }
  
    @override
    void visitRuleSet(RuleSet node) {
      _shimPolyfillUnscopedRule(node);
      super.visitRuleSet(node);
    }
  
    @override
    void visitSelectorGroup(SelectorGroup node) {
      if (!_unscopedSelectorGroups.contains(node)) {
        super.visitSelectorGroup(node);
      }
    }
  
    @override
    void visitStyleSheet(StyleSheet node) {
      _shimPolyfillNextSelector(node.topLevels);
      super.visitStyleSheet(node);
    }
  }
  