Migrate async tests to strong

tests/lib_strong/async/future_or_only_in_async_test.dart

+// Copyright (c) 2016, 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.
+
+// `FutureOr<T>` is only visible when `dart:async` is imported.
+
+dynamic foo(dynamic x) {
+  return x as
+      FutureOr< // //# 00: runtime error, static type warning
+          int
+      > //         //# 00: continued
+      ;
+}
+
+main() {
+  if (499 != foo(499)) throw "bad";
+}