[cfe] Add token_leak_dart2js_test

This uses `token_leak_test.dart` to check that we can compile dart2js
without leaking tokens.

Change-Id: I78a419d722a4b017f1bc1e77933f8f54b4cff175
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/330061
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/front_end/test/token_leak_dart2js_test.dart

@@ -0,0 +1,9 @@
+// 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 'token_leak_test.dart' as test;
+
+Future<void> main() async {
+  await test.main(['--no-sdk', 'pkg/compiler/lib/src/dart2js.dart']);
+}