Fix vm_timeline_flags_test flake

runtime/observatory/tests/service/vm_timeline_flags_test.dart

Index: runtime/observatory/tests/service/vm_timeline_flags_test.dart
diff --git a/runtime/observatory/tests/service/vm_timeline_flags_test.dart b/runtime/observatory/tests/service/vm_timeline_flags_test.dart
index 6943874d6e8f55cd6878a8ccd450dfe5ec79f62b..775e40a9879a115c0c212395e57e2c4612b70233 100644
--- a/runtime/observatory/tests/service/vm_timeline_flags_test.dart
+++ b/runtime/observatory/tests/service/vm_timeline_flags_test.dart
@@ -63,6 +63,7 @@ var tests = [
     print(flags);
   },
   resumeIsolate,
+  hasStoppedAtBreakpoint,
   (Isolate isolate) async {
     // Get the timeline.
     Map result = await isolate.vm.invokeRpcNoUpgrade('_getVMTimeline', {});
@@ -96,6 +97,7 @@ var tests = [
     expect(flags['recordedStreams'].length, equals(0));
   },
   resumeIsolate,
+  hasStoppedAtBreakpoint,
   (Isolate isolate) async {
     // Grab the timeline and verify that we haven't added any new Dart events.
     Map result = await isolate.vm.invokeRpcNoUpgrade('_getVMTimeline', {});