Fix two bugs with async stack traces.

tests/language/await_future_test.dart

Index: tests/language/await_future_test.dart
diff --git a/tests/language/await_future_test.dart b/tests/language/await_future_test.dart
index 2ec30fe8195cd8991c8b1ae78063a04f82438e6c..8a03792e1c407b4e58baeabe8b957e7afb8d39e5 100644
--- a/tests/language/await_future_test.dart
+++ b/tests/language/await_future_test.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 // VMOptions=--optimization-counter-threshold=5 --no-background-compilation
+// VMOptions=--optimization-counter-threshold=5 --no-background-compilation --stress-async-stacks
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ -233,4 +234,4 @@ main() {
   test().then((_) {
     asyncEnd();
   });
-}
+}