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

sdk/lib/_internal/pub_generated/test/transformer/prefers_transformer_to_library_name_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 WRONG_TRANSFORMER = """
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-class RewriteTransformer extends Transformer {
-  RewriteTransformer.asPlugin();
-
-  String get allowedExtensions => '.txt';
-
-  Future apply(Transform transform) {
-    return transform.primaryInput.readAsString().then((contents) {
-      var id = transform.primaryInput.id.changeExtension(".wrong");
-      transform.addOutput(new Asset.fromString(id, "\$contents.wrong"));
-    });
-  }
-}
-""";
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("prefers transformer.dart to <package name>.dart", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp"]
-        }),
-            d.dir(
-                "lib",
-                [
-                    d.file("transformer.dart", REWRITE_TRANSFORMER),
-                    d.file("myapp.dart", WRONG_TRANSFORMER)]),
-            d.dir("web", [d.file("foo.txt", "foo")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      pubServe();
-      requestShouldSucceed("foo.out", "foo.out");
-      requestShould404("foo.wrong");
-      endPubServe();
-    });
-  });
-}