Fix two bugs with async stack traces.

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
 
 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 = 29;
+const LINE_A = 36;
 
 class Foo {
 
 }
 
 doThrow() {
   throw "TheException"; // Line 13.
   return "end of doThrow";
 }
 
 asyncThrower() async {
   doThrow();
 }
 
 testeeMain() async {
+  // Trigger optimization via OSR.
+  var s = 0;
+  for (var i = 0; i < 100; i++) {
+    s += i;
+  }
+  print(s);
   // No try ... catch.
-  await asyncThrower();
+  await asyncThrower();  // LINE_A
 }
 
 var tests = [
   hasStoppedWithUnhandledException,
 
   (Isolate isolate) async {
     print("We stoppped!");
     var stack = await isolate.getStack();
     expect(stack['asyncCausalFrames'], isNotNull);
     var asyncStack = stack['asyncCausalFrames'];
     expect(asyncStack[0].toString(), contains('doThrow'));
     expect(asyncStack[1].toString(), contains('asyncThrower'));
     expect(asyncStack[2].kind, equals(M.FrameKind.asyncSuspensionMarker));
     expect(asyncStack[3].toString(), contains('testeeMain'));
     // 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);