Detect unhandled exceptions in async functions without an awaiter

runtime/observatory/tests/service/awaiter_async_stack_contents_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
+// VMOptions=--error_on_bad_type --error_on_bad_override  --verbose_debug --async_debugger
 
 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_C = 19;
 const LINE_A = 24;
@@ skipping 19 matching lines @@
   hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_B),
   (Isolate isolate) async {
     ServiceMap stack = await isolate.getStack();
     // No awaiter frames because we are in a completely synchronous stack.
     expect(stack['awaiterFrames'], isNull);
   },
   resumeIsolate,
   hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_A),
-  (Isolate isolate) async {
-    ServiceMap stack = await isolate.getStack();
-    // No awaiter frames because there is no awaiter.
-    expect(stack['awaiterFrames'], isNull);
-  },
   resumeIsolate,
   hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_C),
   (Isolate isolate) async {
     // Verify awaiter stack trace is the current frame + the awaiter.
     ServiceMap stack = await isolate.getStack();
     expect(stack['awaiterFrames'], isNotNull);
     List awaiterFrames = stack['awaiterFrames'];
     expect(awaiterFrames.length, greaterThanOrEqualTo(4));
     // Awaiter frame.
     expect(await awaiterFrames[0].toUserString(),
         stringContainsInOrder(['foobar', '.dart:19']));
     // Awaiter frame.
     expect(await awaiterFrames[1].toUserString(),
         stringContainsInOrder(['helper', '.dart:25']));
     // Suspension point.
     expect(awaiterFrames[2].kind, equals(M.FrameKind.asyncSuspensionMarker));
     // Causal frame.
     expect(await awaiterFrames[3].toUserString(),
         stringContainsInOrder(['testMain', '.dart:30']));
   },
 ];
 
 main(args) =>
     runIsolateTestsSynchronous(args, tests, testeeConcurrent: testMain);