Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/transformer/fails_to_load_a_pubspec_with_reserved_transformer_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 'package:scheduled_test/scheduled_test.dart';
-
-import '../../lib/src/exit_codes.dart' as exit_codes;
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../serve/utils.dart';
-
-main() {
-  initConfig();
-
-  withBarbackVersions("any", () {
-    integration("fails to load a pubspec with reserved transformer", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["\$nonexistent"]
-        }),
-            d.dir(
-                "lib",
-                [d.dir("src", [d.file("transformer.dart", REWRITE_TRANSFORMER)])])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      var pub = startPubServe();
-      pub.stderr.expect(
-          contains(
-              'Invalid transformer config: Unsupported '
-                  'built-in transformer \$nonexistent.'));
-      pub.shouldExit(exit_codes.DATA);
-    });
-  });
-}