Mark service/async_scope_test as slow on windows+checked+debug.

runtime/observatory/tests/service/service.status

Index: runtime/observatory/tests/service/service.status
diff --git a/runtime/observatory/tests/service/service.status b/runtime/observatory/tests/service/service.status
index d6b949af3cfdae57c70ab60666cf2a67ee94e2c6..cfb9be9e93972444adbae52ebb9e2ae427689518 100644
--- a/runtime/observatory/tests/service/service.status
+++ b/runtime/observatory/tests/service/service.status
@@ -11,6 +11,9 @@ pause_on_start_then_step_test: Pass, RuntimeError # Issue 28624
 isolate_lifecycle_test: Pass, RuntimeError # Issue 24174
 field_script_test: Pass, RuntimeError
 
+[ $system == windows && $mode == debug && $checked ]
+async_scope_test: Pass, Slow
+
 [$runtime == vm && $compiler == none && $system == fuchsia]
 *: Skip  # Not yet triaged.