Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/serve/missing_dependency_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';
-
-main() {
-  initConfig();
-  integration("responds with a 404 for a missing files in dependencies", () {
-    d.dir("foo", [d.libPubspec("foo", "0.0.1")]).create();
-
-    d.dir(appPath, [d.appPubspec({
-        "foo": {
-          "path": "../foo"
-        }
-      })]).create();
-
-    pubGet();
-    pubServe();
-    requestShould404("packages/foo/nope.dart");
-    requestShould404("dir/packages/foo/nope.dart");
-    endPubServe();
-  });
-}