Rewrite asyncStepOver to use await

runtime/observatory/tests/service/smart_next_test.dart

Index: runtime/observatory/tests/service/smart_next_test.dart
diff --git a/runtime/observatory/tests/service/smart_next_test.dart b/runtime/observatory/tests/service/smart_next_test.dart
index aeb819ff471fab77345fb10f4a1ffabad0641656..34bac34a5b930f715bdbf028fb28cccf0a9400f2 100644
--- a/runtime/observatory/tests/service/smart_next_test.dart
+++ b/runtime/observatory/tests/service/smart_next_test.dart
@@ -47,7 +47,8 @@ stepOverAwaitingResume(Isolate isolate) async {
 smartNext(Isolate isolate) async {
   if (isolate.pauseEvent.atAsyncJump) {
     print("next-async");
-    return isolate.asyncStepOver()[Isolate.kSecondResume];
+    List asyncStepFutures = await isolate.asyncStepOver();
+    return asyncStepFutures[Isolate.kSecondResume];
   } else {
     print("next-sync");
     return stepOverAwaitingResume(isolate);