Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/global/run/runs_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';
-
-const TRANSFORMER = """
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-class DartTransformer extends Transformer {
-  DartTransformer.asPlugin();
-
-  String get allowedExtensions => '.in';
-
-  void apply(Transform transform) {
-    transform.addOutput(new Asset.fromString(
-        new AssetId("foo", "bin/script.dart"),
-        "void main() => print('generated');"));
-  }
-}
-""";
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration('runs a global script generated by a transformer', () {
-      makeGlobalPackage("foo", "1.0.0", [d.pubspec({
-          "name": "foo",
-          "version": "1.0.0",
-          "transformers": ["foo/src/transformer"]
-        }),
-            d.dir(
-                "lib",
-                [
-                    d.dir(
-                        "src",
-                        [d.file("transformer.dart", TRANSFORMER), d.file("primary.in", "")])])],
-            pkg: ['barback']);
-
-      var pub = pubRun(global: true, args: ["foo:script"]);
-
-      pub.stdout.expect("generated");
-      pub.shouldExit();
-    });
-  });
-}