Fix a couple of bugs with async stack traces

runtime/observatory/tests/service/regress_28443_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.
 
 import 'dart:async';
 import 'test_helper.dart';
 import 'service_test_common.dart';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 13 matching lines @@
       vmService = new VMServiceClient(uri);
       await new Future.microtask(() => throw new TimeoutException("here"));
     } on dynamic {
       vmService.close();
       rethrow; // LINE_A
     }
   });
 }
 
 test_code() async {
-  // LINE_B
   try {
     await collect();
   } on TimeoutException {
     print("ok");
   }
 }
 
 Future<Isolate> stepThroughProgram(Isolate isolate) async {
   Completer completer = new Completer();
   int pauseEventsSeen = 0;
@@ skipping 25 matching lines @@
   setBreakpointAtLine(LINE_A),
   resumeIsolate,
   hasStoppedAtBreakpoint,
   stepOut,
   stoppedAtLine(LINE_B),
   resumeIsolate
 ];
 
 main(args) => runIsolateTestsSynchronous(args, tests,
     testeeConcurrent: test_code, pause_on_start: true, pause_on_exit: false);