[ddc] Skip async/return_throw_test on firefox
[ddc] Skip async/return_throw_test on firefox

Appears to be consistently failing on all other backends.

Re-enable this test if it is decided that this should be the
correct behavior.

Change-Id: If5016d23954748c0cb193f2f5faedb9f12820b5f
Issue: https://github.com/dart-lang/sdk/issues/44395
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214481
Reviewed-by: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>
Commit-Queue: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk