Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/run/doesnt_load_an_unnecessary_transformer_test.dart

-// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS 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.
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../serve/utils.dart';
-
-const TRANSFORMER = """
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-class BrokenTransformer extends Transformer {
-  RewriteTransformer.asPlugin() {
-    throw 'This transformer is broken!';
-  }
-
-  String get allowedExtensions => '.txt';
-
-  void apply(Transform transform) {}
-}
-""";
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("doesn't load an unnecessary transformer", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": [{
-              "myapp/src/transformer": {
-                r"$include": "lib/myapp.dart"
-              }
-            }]
-        }),
-            d.dir(
-                "lib",
-                [
-                    d.file("myapp.dart", ""),
-                    d.dir("src", [d.file("transformer.dart", TRANSFORMER)])]),
-            d.dir("bin", [d.file("hi.dart", "void main() => print('Hello!');")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      // This shouldn't load the transformer, since it doesn't transform
-      // anything that the entrypoint imports. If it did load the transformer,
-      // we'd know since it would throw an exception.
-      var pub = pubRun(args: ["hi"]);
-      pub.stdout.expect("Hello!");
-      pub.shouldExit();
-    });
-  });
-}