anchor.diff:    ConstraintVariable possiblyOptionalParameter(String text) {
       return _variables
           .possiblyOptionalParameter(findNode.defaultParameter(text));
  +  }
  +
  +  @override
  +  void setUp() {
  +    createAnalysisOptionsFile(experiments: [EnableString.non_nullable]);
  +    super.setUp();
     }
   
     /// Gets the [ConditionalDiscard] information associated with the statement
  
positive.code: import 'package:analysis_server/src/nullability/transitional_api.dart';
  import 'package:analysis_server/src/nullability/unit_propagation.dart';
  import 'package:analyzer/dart/ast/ast.dart';
  import 'package:analyzer/src/generated/resolver.dart';
  import 'package:analyzer/src/generated/source.dart';
  import 'package:analyzer/src/test_utilities/find_node.dart';
  
negative.code:       _variables.decoratedElementType(
            findNode.functionDeclaration(search).declaredElement);
  
    test_interfaceType_nullable() async {
      await analyze('''
  void f(int? x) {}
  