Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/transformer/loads_a_transformer_defined_in_an_exported_library_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 '../serve/utils.dart';
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("loads a transformer defined in an exported library", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp"]
-        }),
-            d.dir(
-                "lib",
-                [
-                    d.file("myapp.dart", "export 'src/transformer.dart';"),
-                    d.dir("src", [d.file("transformer.dart", REWRITE_TRANSFORMER)])]),
-            d.dir("web", [d.file("foo.txt", "foo")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      pubServe();
-      requestShouldSucceed("foo.out", "foo.out");
-      endPubServe();
-    });
-  });
-}