Fix test failure and add new awaiterFrames to service_dev.md

runtime/observatory/tests/service/async_single_step_exception_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:async';
 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;
-const LINE_F = 35;
-const LINE_G = 38;
+const LINE_A = 23;
+const LINE_B = 24;
+const LINE_C = 30;
+const LINE_D = 32;
+const LINE_E = 35;
+const LINE_F = 38;
+const LINE_G = 40;
 
 helper() async {
   print('helper'); // LINE_A.
   throw 'a'; // LINE_B.
+  return null;
 }
 
 testMain() async {
   debugger();
   print('mmmmm'); // LINE_C.
   try {
     await helper(); // LINE_D.
   } catch (e) {
     // arrive here on error.
     print('error: $e'); // LINE_E.
@@ skipping 11 matching lines @@
   hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_D),
   stepInto,
   hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_A),
   stepOver, // print.
   hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_B), // throw 'a'.
   stepInto, // exit helper via a throw.
   hasStoppedAtBreakpoint,
+  stepInto, // exit helper via a throw.
+  hasStoppedAtBreakpoint,
+  stepInto, // step once from entry to main.
+  hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_E), // print(error)
   stepOver,
   hasStoppedAtBreakpoint,
+  stepOver,
+  hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_F), // print(foo)
   stepOver,
   hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_G), // print(z)
   resumeIsolate
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testMain);