Add heart beat tests to make sure we see an isolate's sticky error after exit

runtime/observatory/tests/service/service_test_common.dart

Index: runtime/observatory/tests/service/service_test_common.dart
diff --git a/runtime/observatory/tests/service/service_test_common.dart b/runtime/observatory/tests/service/service_test_common.dart
index 108316c3030fb0f056c5157662ed0cc5d57fe347..4a42290543cc00e9d231d212d3e0e76de7dc30b0 100644
--- a/runtime/observatory/tests/service/service_test_common.dart
+++ b/runtime/observatory/tests/service/service_test_common.dart
@@ -140,6 +140,10 @@ Future<Isolate> hasStoppedWithUnhandledException(Isolate isolate) {
   return hasPausedFor(isolate, ServiceEvent.kPauseException);
 }
 
+Future<Isolate> hasStoppedAtExit(Isolate isolate) {
+  return hasPausedFor(isolate, ServiceEvent.kPauseExit);
+}
+
 Future<Isolate> hasPausedAtStart(Isolate isolate) {
   return hasPausedFor(isolate, ServiceEvent.kPauseStart);
 }