Migrate async tests to strong

tests/lib_strong/async/timer_not_available_test.dart

+// Copyright (c) 2011, 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.
+
+library timerNotAvailable;
+
+import 'package:expect/expect.dart';
+import 'dart:async';
+
+main() {
+  final ms = const Duration(milliseconds: 1);
+  bool failed = false;
+  try {
+    new Timer(ms * 5, () { });
+  } on UnsupportedError catch (e) {
+    failed = true;
+  }
+  Expect.isTrue(failed);
+  failed = false;
+  try {
+    var t = new Timer.periodic(ms * 10, (_) { });
+    t.cancel();
+  } on UnsupportedError catch (e) {
+    failed = true;
+  }
+  Expect.isTrue(failed);
+}