Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/serve/native_watch_replaced_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 'dart:async';
-import 'dart:io';
-
-import 'package:path/path.dart' as p;
-import 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import 'utils.dart';
-
-main() {
-  // This is a regression test for http://dartbug.com/21402.
-  initConfig();
-  withBarbackVersions("any", () {
-    integration(
-        "picks up files replaced after serving started when using the "
-            "native watcher",
-        () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
-        }),
-            d.dir("lib", [d.dir("src", [d.file("transformer.dart", REWRITE_TRANSFORMER)])]),
-            d.dir("web", [d.file("file.txt", "before"),]),
-            d.file("other", "after")]).create();
-
-      createLockFile("myapp", pkg: ["barback"]);
-
-      pubServe(args: ["--no-force-poll"]);
-      waitForBuildSuccess();
-      requestShouldSucceed("file.out", "before.out");
-
-      schedule(() {
-        // Replace file.txt by renaming other on top of it.
-        return new File(
-            p.join(
-                sandboxDir,
-                appPath,
-                "other")).rename(p.join(sandboxDir, appPath, "web", "file.txt"));
-      });
-
-      // Read the transformed file to ensure the change is actually noticed by
-      // pub and not that we just get the new file contents piped through
-      // without pub realizing they've changed.
-      waitForBuildSuccess();
-      requestShouldSucceed("file.out", "after.out");
-
-      endPubServe();
-    });
-  });
-}