anchor.diff:  }
   
   abstract class MemberVisitor<R> {
  +  const MemberVisitor();
  +
     R defaultMember(Member node) => null;
   
     R visitConstructor(Constructor node) => defaultMember(node);
  
positive.code: import 'ast.dart';
  
  abstract class ExpressionVisitor<R> {
    R defaultExpression(Expression node) => null;
    R defaultBasicLiteral(BasicLiteral node) => defaultExpression(node);
  
  
negative.code: }
  
  abstract class StatementVisitor1<R, T> {
    const StatementVisitor1();
  
    R defaultStatement(Statement node, T arg) => null;
  
    R visitInvalidStatement(InvalidStatement node, T arg) =>
  