Migrate async tests to strong

tests/lib_strong/async/stack_trace12_test.dart

+// Copyright (c) 2013, 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.
+
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
+import 'dart:async';
+
+main() {
+  StackTrace trace;
+  asyncStart();
+  var f = new Future(() { throw "foo"; });
+  f.then((_) { throw "Unreachable"; },
+    onError: (e, st) {
+      Expect.equals("foo", e);
+      Expect.isNotNull(st);
+      asyncEnd();
+    });
+}