Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/serve/native_watch_modified_file_test.dart

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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';
-import 'utils.dart';
-
-// TODO(nweiz): Default to testing the native watcher and add an explicit test
-// for the polling watcher when issue 14941 is fixed.
-
-main() {
-  initConfig();
-  integration(
-      "watches modifications to files when using the native watcher",
-      () {
-    d.dir(
-        appPath,
-        [d.appPubspec(), d.dir("web", [d.file("index.html", "before")])]).create();
-
-    pubServe(args: ["--no-force-poll"]);
-    requestShouldSucceed("index.html", "before");
-
-    d.dir(appPath, [d.dir("web", [d.file("index.html", "after")])]).create();
-
-    waitForBuildSuccess();
-    requestShouldSucceed("index.html", "after");
-    endPubServe();
-  });
-}