anchor.diff:          .add(new _Subscription(linter, visitor, _getTimer(linter)));
     }
   
  +  void addMixinDeclaration(LintRule linter, AstVisitor visitor) {
  +    _forMixinDeclaration
  +        .add(new _Subscription(linter, visitor, _getTimer(linter)));
  +  }
  +
     void addNamedExpression(LintRule linter, AstVisitor visitor) {
       _forNamedExpression
           .add(new _Subscription(linter, visitor, _getTimer(linter)));
  
positive.code:   }
  
    @override
    void visitNamedExpression(NamedExpression node) {
      _runSubscriptions(node, registry._forNamedExpression);
      super.visitNamedExpression(node);
  
negative.code:   void visitNullLiteral(NullLiteral node) {
      _runSubscriptions(node, registry._forNullLiteral);
      super.visitNullLiteral(node);
    }
  
    @override
    void visitOnClause(OnClause node) {
      _runSubscriptions(node, registry._forOnClause);
      super.visitOnClause(node);
    }
  
    @override
  