Use native async/await support in pub.

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

-// Copyright (c) 2014, 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();
-  withBarbackVersions("any", () {
-    integration("supports a user-defined lazy transformer", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
-        }),
-            d.dir("lib", [d.dir("src", [d.file("transformer.dart", LAZY_TRANSFORMER)])]),
-            d.dir("web", [d.file("foo.txt", "foo")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      var server = pubServe();
-      // The build should complete without the transformer logging anything.
-      server.stdout.expect('Build completed successfully');
-
-      requestShouldSucceed("foo.out", "foo.out");
-      server.stdout.expect(
-          emitsLines('[Info from LazyRewrite]:\n' 'Rewriting myapp|web/foo.txt.'));
-      endPubServe();
-    });
-  });
-}