Add tests for `FutureOr<T>`.

tests/lib/async/future_or_only_in_async_test.dart

Index: tests/lib/async/future_or_only_in_async_test.dart
diff --git a/tests/lib/async/future_or_only_in_async_test.dart b/tests/lib/async/future_or_only_in_async_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..3e7d098a799f5ee6354a26c658288604c3edbf42
--- /dev/null
+++ b/tests/lib/async/future_or_only_in_async_test.dart
@@ -0,0 +1,17 @@
+// 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";
+}