Migrate async tests to strong

tests/lib_strong/async/wait_for_cancel_test.dart

+// Copyright (c) 2015, 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 "dart:async";
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+main() {
+  asyncStart();
+
+  bool waitedForCancel = false;
+
+  var controller = new StreamController(
+      onCancel: () => new Future(() => waitedForCancel = true));
+  var sub = controller.stream.take(1).listen((x) {
+    Expect.fail("onData should not be called");
+  });
+  var cancelFuture = sub.cancel();
+  Expect.isNotNull(cancelFuture);
+  cancelFuture.then((_) {
+    Expect.isTrue(waitedForCancel);
+    asyncEnd();
+  });
+}