Use native async/await support in pub.

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

-// Copyright (c) 2012, 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 '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration(
-      "downgrades a locked package's dependers in order to get it to " "min version",
-      () {
-    servePackages((builder) {
-      builder.serve("foo", "2.0.0", deps: {
-        "bar": ">1.0.0"
-      });
-      builder.serve("bar", "2.0.0");
-    });
-
-    d.appDir({
-      "foo": "any",
-      "bar": "any"
-    }).create();
-
-    pubGet();
-
-    d.packagesDir({
-      "foo": "2.0.0",
-      "bar": "2.0.0"
-    }).validate();
-
-    servePackages((builder) {
-      builder.serve("foo", "1.0.0", deps: {
-        "bar": "any"
-      });
-      builder.serve("bar", "1.0.0");
-    });
-
-    pubDowngrade(args: ['bar']);
-
-    d.packagesDir({
-      "foo": "1.0.0",
-      "bar": "1.0.0"
-    }).validate();
-  });
-}