Add tests for `FutureOr<T>`.

tests/lib/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.
+
+T foo<T>(T x) {
+  return x as
+      FutureOr<  /// 00: runtime error, static type warning
+        T
+      >          /// 00: continued
+  ;
+}
+
+main() {
+  if (499 != foo<int>(499)) throw "bad";
+}