[gardening] Mark service/pause_on_unhandled_async_exceptions2_test with dartk slow.

Change-Id: I423649e2232dfbb52c7da2a72fee749484af3f14
Reviewed-on: https://dart-review.googlesource.com/c/78325
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>
diff --git a/runtime/observatory/tests/service/service_kernel.status b/runtime/observatory/tests/service/service_kernel.status
index f702437..147798d 100644
--- a/runtime/observatory/tests/service/service_kernel.status
+++ b/runtime/observatory/tests/service/service_kernel.status
@@ -112,6 +112,7 @@
 evaluate_activation_in_method_class_test: RuntimeError
 evaluate_activation_test/instance: RuntimeError # http://dartbug.com/20047
 evaluate_activation_test/scope: RuntimeError # http://dartbug.com/20047
+pause_on_unhandled_async_exceptions_test: Pass, Slow
 step_through_arithmetic_test: RuntimeError # probably constant evaluator pre-evaluating e.g. 1+2
 unused_changes_in_last_reload_test: RuntimeError