Implement support for single stepping out of an async function.

runtime/observatory/tests/service/step_over_await_test.dart

Index: runtime/observatory/tests/service/step_over_await_test.dart
diff --git a/runtime/observatory/tests/service/step_over_await_test.dart b/runtime/observatory/tests/service/step_over_await_test.dart
index f1e8d7ceea177103e7c9e1735d031753fba5e12e..c00cc91b8498740cb7efd48e8f3d67c52bd4575d 100644
--- a/runtime/observatory/tests/service/step_over_await_test.dart
+++ b/runtime/observatory/tests/service/step_over_await_test.dart
@@ -16,7 +16,7 @@ import 'package:unittest/unittest.dart';
 const int LINE_A = 22;
 const int LINE_B = 24;
 
-// This tests the asyncStepOver command.
+// This tests the asyncNext command.
 asyncFunction() async {
   debugger();
   print('a'); // LINE_A
@@ -38,7 +38,8 @@ var tests = [
   (Isolate isolate) async {
     expect(M.isAtAsyncSuspension(isolate.pauseEvent), isTrue);
   },
-  asyncStepOver,
+  asyncNext,
+  hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_B),
   resumeIsolate,
 ];