Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/transformer/fails_to_load_a_transform_with_an_import_error_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 '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../serve/utils.dart';
-
-main() {
-  initConfig();
-
-  // An import error will cause the isolate API to fail synchronously while
-  // loading the transformer.
-  withBarbackVersions("any", () {
-    integration("fails to load a transform with an import error", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
-        }),
-            d.dir(
-                "lib",
-                [
-                    d.dir(
-                        "src",
-                        [d.file("transformer.dart", "import 'does/not/exist.dart';")])])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-      var pub = startPubServe();
-      pub.stderr.expect("Unhandled exception:");
-      pub.stderr.expect(
-          startsWith("Uncaught Error: Load Error: Failure getting "));
-      pub.shouldExit(1);
-    });
-  });
-}