Revert "Revert "Use native async/await support in pub.""

sdk/lib/_internal/pub_generated/test/transformer/fails_to_load_a_non_existent_transform_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';
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("fails to load a non-existent transform", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/transform"]
-        })]).create();
-
-      var pub = startPubServe();
-      pub.stderr.expect(
-          'Transformer library "package:myapp/transform.dart" not found.');
-      pub.shouldExit(1);
-    });
-  });
-}