Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/dart2js/compiles_imported_generated_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.
-
-import 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../serve/utils.dart';
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("compiles a Dart file that imports a generated file to JS", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "version": "0.0.1",
-          "transformers": ["myapp/transformer"]
-        }),
-            d.dir("lib", [d.file("transformer.dart", dartTransformer("munge"))]),
-            d.dir("web", [d.file("main.dart", """
-import "other.dart";
-void main() => print(TOKEN);
-"""), d.file("other.dart", """
-library other;
-const TOKEN = "before";
-""")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      pubServe();
-      requestShouldSucceed("main.dart.js", contains("(before, munge)"));
-      endPubServe();
-    });
-  });
-}