Detect unhandled exceptions in async functions without an awaiter

runtime/observatory/tests/service/pause_on_unhandled_async_exceptions2_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
-// VMOptions=--optimization-counter-threshold=5 --error_on_bad_type --error_on_bad_override
+// VMOptions=--error_on_bad_type --error_on_bad_override --async_debugger
+// VMOptions=--optimization-counter-threshold=5 --error_on_bad_type --error_on_bad_override --async_debugger
 
 import 'package:observatory/service_io.dart';
 import 'package:observatory/models.dart' as M;
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 import 'service_test_common.dart';
 
 const LINE_A = 36;
 
 class Foo {
@@ skipping 35 matching lines @@
     // We've stopped at LINE_A.
     expect(await asyncStack[3].location.toUserString(),
            contains('.dart:$LINE_A'));
   }
 ];
 
 main(args) => runIsolateTests(args,
                               tests,
                               pause_on_unhandled_exceptions: true,
                               testeeConcurrent: testeeMain);