dart-lang#2313. [Extension types] Add nullability tests

Author: sgrekhov

LanguageFeatures/Extension-types/nullability_A01_t01.dart

@@ -0,0 +1,58 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion Let V be an extension type of the form Name<T1, .. Ts>, and let R
+/// be the corresponding instantiated representation type. If R is non-nullable
+/// then V is a proper subtype of Object, and V is non-nullable. Otherwise, V is
+/// a proper subtype of Object?, and V is potentially nullable.
+///
+/// @description Checks that an extension type is not-nullable even if its
+/// representation type is nullable
+/// @author [email protected]
+
+// SharedOptions=--enable-experiment=inline-class
+
+import "dart:async" show FutureOr;
+
+extension type ET1(int? _) {}
+
+extension type ET2(Null _) {}
+
+extension type ET3(dynamic _) {}
+
+extension type ET4(void _) {}
+
+extension type ET5(FutureOr<Null> _) {}
+
+main() {
+  ET1 et11 = null;
+//           ^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+  ET1? et12 = null;
+
+  ET2 et21 = null;
+//           ^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+  ET2? et22 = null;
+
+  ET3 et31 = null;
+//           ^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+  ET3? et32 = null;
+
+  ET4 et41 = null;
+//           ^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+  ET4? et42 = null;
+
+  ET5 et51 = null;
+//           ^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+  ET5? et52 = null;
+}

LanguageFeatures/Extension-types/nullability_A01_t02.dart

@@ -0,0 +1,40 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion Let V be an extension type of the form Name<T1, .. Ts>, and let R
+/// be the corresponding instantiated representation type. If R is non-nullable
+/// then V is a proper subtype of Object, and V is non-nullable. Otherwise, V is
+/// a proper subtype of Object?, and V is potentially nullable.
+///
+/// @description Checks that null can be assigned to an extension type at run
+/// time if its representation type is nullable
+/// @author [email protected]
+
+// SharedOptions=--enable-experiment=inline-class
+
+import "dart:async" show FutureOr;
+import "../../Utils/expect.dart";
+
+extension type ET1(int? _) {}
+
+extension type ET2(Null _) {}
+
+extension type ET3(dynamic _) {}
+
+extension type ET4(void _) {}
+
+extension type ET5(FutureOr<Null> _) {}
+
+main() {
+  ET1 et1 = null as dynamic;
+  Expect.isNull(et1);
+  ET2 et2 = null as dynamic;
+  Expect.isNull(et2);
+  ET3 et3 = null as dynamic;
+  Expect.isNull(et3);
+  ET4 et4 = null as dynamic;
+  Expect.isNull(et4);
+  ET5 et5 = null as dynamic;
+  Expect.isNull(et5);
+}