Implement support for single stepping out of an async function.

runtime/observatory/tests/service/async_single_step_out_test.dart

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+// VMOptions=--error_on_bad_type --error_on_bad_override  --verbose_debug
+
+import 'dart:developer';
+import 'package:observatory/models.dart' as M;
+import 'package:observatory/service_io.dart';
+import 'package:unittest/unittest.dart';
+import 'service_test_common.dart';
+import 'test_helper.dart';
+
+const LINE_A = 20;
+const LINE_B = 21;
+const LINE_C = 26;
+const LINE_D = 27;
+const LINE_E = 28;
+
+helper() async {
+  print('helper'); // LINE_A.
+  return null; // LINE_B.
+}
+
+testMain() async {
+  debugger();
+  print('mmmmm'); // LINE_C.
+  await helper(); // LINE_D.
+  print('z'); // LINE_E.
+}
+
+var tests = [
+  hasStoppedAtBreakpoint,
+  stoppedAtLine(LINE_C),
+  stepOver, // print.
+  hasStoppedAtBreakpoint,
+  stoppedAtLine(LINE_D),
+  stepInto,
+  hasStoppedAtBreakpoint,
+  stoppedAtLine(LINE_A),
+  stepOver, // print.
+  hasStoppedAtBreakpoint,
+  stoppedAtLine(LINE_B), // return null.
+  stepInto, // exit helper via a single step.
+  hasStoppedAtBreakpoint,
+  stoppedAtLine(LINE_E), // arrive after the await.
+  resumeIsolate
+];
+
+main(args) => runIsolateTests(args, tests, testeeConcurrent: testMain);