anchor.diff:  }
   
   class MemberReferenceVisitor<R> {
  +  const MemberReferenceVisitor();
  +
     R defaultMemberReference(Member node) => null;
   
     R visitFieldReference(Field node) => defaultMemberReference(node);
  
positive.code: }
  
  abstract class InitializerVisitor<R> {
    R defaultInitializer(Initializer node) => null;
  
    R visitInvalidInitializer(InvalidInitializer node) =>
  
negative.code:         StatementVisitor<R>,
          MemberVisitor<R>,
          InitializerVisitor<R> {
    const TreeVisitor();
  
    R defaultTreeNode(TreeNode node) => null;
  
    // Expressions
  