Fix a couple of bugs with async stack traces

runtime/observatory/tests/service/async_generator_breakpoint_test.dart

 // Copyright (c) 2015, 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 --stacktrace-every=55 --stress-async-stacks
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 
 printSync() {
   print('sync'); // Line 11
 }
 
 printAsync() async {
@@ skipping 15 matching lines @@
   // the breakpoints because we need the event loop to remain
   // operational for the async bodies to run.
   print('testee waiting');
   while (!testerReady);
 
   printSync();
   var future = printAsync();
   var stream = printAsyncStar();
   var iterator = printSyncStar();
 
-  print('middle'); // Line 40
+  print('middle'); // Line 41
 
   future.then((v) => print(v));
   stream.toList();
   iterator.toList();
 }
 
 testAsync(Isolate isolate) async {
   await isolate.rootLibrary.load();
   var script = isolate.rootLibrary.scripts[0];
 
   var bp1 = await isolate.addBreakpoint(script, 11);
   expect(bp1, isNotNull);
   expect(bp1 is Breakpoint, isTrue);
   var bp2 = await isolate.addBreakpoint(script, 15);
   expect(bp2, isNotNull);
   expect(bp2 is Breakpoint, isTrue);
   var bp3 = await isolate.addBreakpoint(script, 19);
   expect(bp3, isNotNull);
   expect(bp3 is Breakpoint, isTrue);
   var bp4 = await isolate.addBreakpoint(script, 23);
   expect(bp4, isNotNull);
   expect(bp4 is Breakpoint, isTrue);
-  var bp5 = await isolate.addBreakpoint(script, 40);
+  var bp5 = await isolate.addBreakpoint(script, 41);
   print("BP5 - $bp5");
   expect(bp5, isNotNull);
   expect(bp5 is Breakpoint, isTrue);
 
   var hits = [];
 
   isolate.rootLibrary.evaluate('testerReady = true;').then((Instance result) {
     expect(result.valueAsString, equals('true'));
   });
 
   var stream = await isolate.vm.getEventStream(VM.kDebugStream);
   await for (ServiceEvent event in stream) {
     if (event.kind == ServiceEvent.kPauseBreakpoint) {
       var bp = event.breakpoint;
       print('Hit $bp');
       hits.add(bp);
       await isolate.resume();
 
       if (hits.length == 5) break;
     }
   }
 
   expect(hits, equals([bp1, bp5, bp4, bp2, bp3]));
 }
 
 var tests = [testAsync];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testeeDo);