Fix async_generator_breakpoint_test so it awaits on isolate resume requests

runtime/observatory/tests/service/async_generator_breakpoint_test.dart

Index: runtime/observatory/tests/service/async_generator_breakpoint_test.dart
diff --git a/runtime/observatory/tests/service/async_generator_breakpoint_test.dart b/runtime/observatory/tests/service/async_generator_breakpoint_test.dart
index 7a26e42e745834d1686877d53196db84273e2243..aef2e8e3e973041575eed36740845609c2fe90da 100644
--- a/runtime/observatory/tests/service/async_generator_breakpoint_test.dart
+++ b/runtime/observatory/tests/service/async_generator_breakpoint_test.dart
@@ -75,7 +75,7 @@ testAsync(Isolate isolate) async {
       var bp = event.breakpoint;
       print('Hit $bp');
       hits.add(bp);
-      isolate.resume();
+      await isolate.resume();
 
       if (hits.length == 5) break;
     }