Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/transformer/runs_a_transformer_group_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 '../serve/utils.dart';
-
-const GROUP = """
-import 'package:barback/barback.dart';
-
-import 'transformer.dart';
-
-class RewriteGroup implements TransformerGroup {
-  RewriteGroup.asPlugin();
-
-  Iterable<Iterable> get phases => [[new RewriteTransformer.asPlugin()]];
-}
-""";
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("runs a transformer group", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/group"]
-        }),
-            d.dir(
-                "lib",
-                [
-                    d.dir(
-                        "src",
-                        [
-                            d.file("transformer.dart", REWRITE_TRANSFORMER),
-                            d.file("group.dart", GROUP)])]),
-            d.dir("web", [d.file("foo.txt", "foo")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      pubServe();
-      requestShouldSucceed("foo.out", "foo.out");
-      endPubServe();
-    });
-  });
-}