anchor.diff:  import 'package:kernel/ast.dart'
       show
           AsExpression,
  +        BottomType,
           Class,
           Component,
           DartType,
  +        DynamicType,
           ExpressionStatement,
           Field,
           FunctionType,
           InterfaceType,
  +        InvalidType,
           Library,
           Member,
  +        NeverType,
           Nullability,
           Procedure,
           StaticInvocation,
  
positive.code: 
    @override
    visitInterfaceType(InterfaceType node) {
      Uri importUri = node.classNode.enclosingLibrary.importUri;
      if (node.classNode.name == "Null" &&
          importUri.scheme == "dart" &&
          importUri.path == "core" &&
          node.nullability != Nullability.nullable) {
        problem(null, "Found a not nullable Null type: ${node}");
      }
      super.visitInterfaceType(node);
  
negative.code: 
  class FastaVerifyingVisitor extends VerifyingVisitor {
    final List<LocatedMessage> errors = <LocatedMessage>[];
    Library currentLibrary = null;
  
    Uri fileUri;
    final bool skipPlatform;
  