Migrate async tests to strong

tests/lib_strong/async/zone_root_bind_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() {
+  Expect.identical(Zone.ROOT, Zone.current);
+  Zone forked = Zone.current.fork();
+  var f = Zone.current.bindCallback(() {
+    Expect.identical(Zone.ROOT, Zone.current);
+  }, runGuarded: false);
+  forked.run(() {
+    f();
+  });
+}