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

sdk/lib/_internal/pub_generated/test/serve/allows_arbitrary_modes_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 TRANSFORMER = """
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-class ModeTransformer extends Transformer {
-  final BarbackSettings settings;
-  ModeTransformer.asPlugin(this.settings);
-
-  String get allowedExtensions => '.txt';
-
-  Future apply(Transform transform) {
-    return new Future.value().then((_) {
-      var id = transform.primaryInput.id.changeExtension(".out");
-      transform.addOutput(new Asset.fromString(id, settings.mode.toString()));
-    });
-  }
-}
-""";
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("allows user-defined mode names", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
-        }),
-            d.dir("lib", [d.dir("src", [d.file("transformer.dart", TRANSFORMER)])]),
-            d.dir("web", [d.file("foo.txt", "foo")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      pubServe(args: ["--mode", "depeche"]);
-      requestShouldSucceed("foo.out", "depeche");
-      endPubServe();
-    });
-  });
-}