Add test demonstrating that FutureOr<A> <: FutureOr<B> =/=> A <: B.

Change-Id: Iedc60b57f80e77784e54b8d5ed9076c5016edd2f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121653
Commit-Queue: Mayank Patke <fishythefish@google.com>
Reviewed-by: Leaf Petersen <leafp@google.com>

tests/language_2/subtyping_static/future_or_subtype_test.dart

@@ -0,0 +1,16 @@
+// Copyright (c) 2019, 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 'dart:async';
+import 'package:expect/expect.dart';
+
+class A implements Future<Future<A>> {
+  @override
+  noSuchMethod(Invocation _) {}
+}
+
+void main() {
+  Expect.notSubtype<A, Future<A>>();
+  Expect.subtype<FutureOr<A>, FutureOr<Future<A>>>();
+}