[dart2wasm] Fix nullability of type parameter types

Fixes #53968

Change-Id: I3d4d0ff2d0fc9b196322f967600bf392124a75de
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/339400
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

pkg/dart2wasm/lib/translator.dart

@@ -570,7 +570,7 @@ class Translator with KernelNodes {
     }
     if (type is TypeParameterType) {
       return translateStorageType(nullable
-          ? type.bound.withDeclaredNullability(type.nullability)
+          ? type.bound.withDeclaredNullability(Nullability.nullable)
           : type.bound);
     }
     if (type is IntersectionType) {

tests/language/extension_type/regress_53968_test.dart

@@ -0,0 +1,17 @@
+// 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.
+
+// SharedOptions=--enable-experiment=inline-class
+
+// Regression check for https://dartbug.com/53968
+
+extension type A(int a) {}
+
+T returnA<T extends A?>() {
+  return null as T;
+}
+
+void main() {
+  returnA();
+}