Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/get/dry_run_does_not_apply_changes_test.dart

-// Copyright (c) 2014, 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 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("--dry-run shows but does not apply changes", () {
-    servePackages((builder) {
-      builder.serve("foo", "1.0.0");
-    });
-
-    d.appDir({
-      "foo": "1.0.0"
-    }).create();
-
-    pubGet(
-        args: ["--dry-run"],
-        output: allOf(
-            [contains("+ foo 1.0.0"), contains("Would change 1 dependency.")]));
-
-    d.dir(appPath, [// The lockfile should not be created.
-      d.nothing("pubspec.lock"),
-          // The "packages" directory should not have been generated.
-      d.nothing("packages")]).validate();
-  });
-}