Migrate async tests to strong

tests/lib_strong/async/future_value_chain4_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';
+
+class MyFuture implements Future {
+  then(valueHandler, {onError}) {
+    scheduleMicrotask(() { valueHandler(499); });
+  }
+  catchError(_, {test}) => null;
+  whenComplete(_) => null;
+  asStream() => null;
+  timeout(Duration timeLimit, {void onTimeout()}) => null;
+}
+
+main() {
+  asyncStart();
+  Completer completer = new Completer();
+  completer.complete(new MyFuture());
+  Expect.isTrue(completer.isCompleted);
+  Expect.throws(() => completer.complete(42));
+  completer.future.then((_) => asyncEnd());
+}