anchor.diff:  
   class FastaVerifyingVisitor extends VerifyingVisitor {
     final List<LocatedMessage> errors = <LocatedMessage>[];
  +  Library currentLibrary = null;
   
     Uri fileUri;
     final bool skipPlatform;
  
positive.code:   visitProcedure(Procedure node) {
      fileUri = checkLocation(node, node.name.name, node.fileUri);
      super.visitProcedure(node);
    }
  
    @override
  
negative.code: 
    @override
    visitInterfaceType(InterfaceType node) {
      if (isNullType(node) && node.nullability != Nullability.nullable) {
        problem(null, "Found a not nullable Null type: ${node}");
      }
      super.visitInterfaceType(node);
  