Migrate language_2/subtyping_static to NNBD.

Change-Id: I94ea79417ae39f64f558d55489000d720c3191e1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151092
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>

tests/language/subtyping_static/README.md

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+# Feature tests for static analysis involving subtyping
+
+This directory was created in order to hold tests pertaining to the
+static analysis of Dart where it involves subtyping.

tests/language/subtyping_static/future_or_subtype_test.dart

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+// 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>>>();
+}