Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/upgrade/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:path/path.dart' as path;
-import 'package:scheduled_test/scheduled_test.dart';
-
-import '../../lib/src/io.dart';
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("--dry-run shows report but does not apply changes", () {
-    servePackages((builder) {
-      builder.serve("foo", "1.0.0");
-      builder.serve("foo", "2.0.0");
-    });
-
-    // Create the first lockfile.
-    d.appDir({
-      "foo": "1.0.0"
-    }).create();
-
-    pubGet();
-
-    // Change the pubspec.
-    d.appDir({
-      "foo": "any"
-    }).create();
-
-    // Also delete the "packages" directory.
-    schedule(() {
-      deleteEntry(path.join(sandboxDir, appPath, "packages"));
-    });
-
-    // Do the dry run.
-    pubUpgrade(
-        args: ["--dry-run"],
-        output: allOf(
-            [contains("> foo 2.0.0 (was 1.0.0)"), contains("Would change 1 dependency.")]));
-
-    d.dir(appPath, [// The lockfile should be unmodified.
-      d.matcherFile("pubspec.lock", contains("1.0.0")),
-          // The "packages" directory should not have been regenerated.
-      d.nothing("packages")]).validate();
-  });
-}