Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/serve/404_page_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 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import 'utils.dart';
-
-main() {
-  initConfig();
-  setUp(() {
-    d.dir(appPath, [d.appPubspec()]).create();
-  });
-
-  integration("the 404 page describes the missing asset", () {
-    pubServe();
-
-    scheduleRequest("packages/foo/missing.txt").then((response) {
-      expect(response.statusCode, equals(404));
-
-      // Should mention the asset that can't be found.
-      expect(response.body, contains("foo"));
-      expect(response.body, contains("missing.txt"));
-    });
-
-    endPubServe();
-  });
-
-  integration("the 404 page describes the error", () {
-    pubServe();
-
-    scheduleRequest("packages").then((response) {
-      expect(response.statusCode, equals(404));
-
-      // Should mention the asset that can't be found.
-      expect(response.body, contains('"/packages"'));
-    });
-
-    endPubServe();
-  });
-}