Fix two bugs with async stack traces.

tests/language/await_test.dart

Index: tests/language/await_test.dart
diff --git a/tests/language/await_test.dart b/tests/language/await_test.dart
index d91c2a831f5fd1e33d3f8602ae3013f12affd49a..59c096850a1cba2359ae6d1e0068cf7f7585d4a1 100644
--- a/tests/language/await_test.dart
+++ b/tests/language/await_test.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 // VMOptions=--optimization-counter-threshold=10 --no-background-compilation
+// VMOptions=--optimization-counter-threshold=10 --no-background-compilation --stress-async-stacks
 
 import 'package:expect/expect.dart';