[tests] Add repro for cross module constant

There is an issue with cross module constant extension types that causes
the CFE to crash.

Change-Id: Id6af3a5400e55ecb2534ce71a07c5c1ecb17a46f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/335384
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

tests/modular/const_extension_type/def.dart

@@ -0,0 +1,7 @@
+// 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.
+
+const val = ExtString('hello');
+
+extension type const ExtString(String s) {}

tests/modular/const_extension_type/main.dart

@@ -0,0 +1,15 @@
+// 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.
+import 'package:expect/expect.dart';
+
+import 'def.dart';
+
+main() {
+  ExtString x = val;
+  fn(x);
+}
+
+void fn(ExtString es) {
+  Expect.equals('hello', es.toString());
+}

tests/modular/const_extension_type/modules.yaml

@@ -0,0 +1,10 @@
+# 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.
+#
+# A constant created with an extension type should be assignable to the same
+# extension type in a different module.
+dependencies:
+  main: [def, expect]
+flags:
+  - inline-class