Migrate async tests to strong

tests/lib_strong/async/future_value_chain_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:async_helper/async_helper.dart';
+import "package:expect/expect.dart";
+import 'dart:async';
+
+
+main() {
+  asyncStart();
+  var errorFuture = new Future.error(499);
+  errorFuture.catchError((x) {
+    Expect.equals(499, x);
+    var valueChainFuture = new Future.value(errorFuture);
+    // The errorFuture must not be propagated immediately as we would otherwise
+    // not have time to catch the error.
+    valueChainFuture.catchError((error) {
+      Expect.equals(499, error);
+      asyncEnd();
+    });
+  });
+}