Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/downgrade/does_not_show_other_versions_test.dart

-// Copyright (c) 2013, 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.
-
-library pub_tests;
-
-import 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("does not show how many other versions are available", () {
-    servePackages((builder) {
-      builder.serve("downgraded", "1.0.0");
-      builder.serve("downgraded", "2.0.0");
-      builder.serve("downgraded", "3.0.0-dev");
-    });
-
-    d.appDir({
-      "downgraded": "3.0.0-dev"
-    }).create();
-
-    pubGet();
-
-    // Loosen the constraints.
-    d.appDir({
-      "downgraded": ">=2.0.0"
-    }).create();
-
-    pubDowngrade(output: contains("downgraded 2.0.0 (was 3.0.0-dev)"));
-  });
-}