Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/global/run/mode_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 {
-  final BarbackSettings _settings;
-
-  DartTransformer.asPlugin(this._settings);
-
-  String get allowedExtensions => '.in';
-
-  void apply(Transform transform) {
-    transform.addOutput(new Asset.fromString(
-        new AssetId(transform.primaryInput.id.package, "bin/script.dart"),
-        "void main() => print('\${_settings.mode.name}');"));
-  }
-}
-""";
-
-main() {
-  initConfig();
-  integration(
-      'runs a script in an activated package with customizable modes',
-      () {
-    servePackages((builder) {
-      builder.serveRepoPackage("barback");
-
-      builder.serve("foo", "1.0.0", deps: {
-        "barback": "any"
-      }, pubspec: {
-        "transformers": ["foo/src/transformer"]
-      },
-          contents: [
-              d.dir(
-                  "lib",
-                  [
-                      d.dir(
-                          "src",
-                          [d.file("transformer.dart", TRANSFORMER), d.file("primary.in", "")])])]);
-    });
-
-    schedulePub(args: ["global", "activate", "foo"]);
-
-    // By default it should run in release mode.
-    var pub = pubRun(global: true, args: ["foo:script"]);
-    pub.stdout.expect("release");
-    pub.shouldExit();
-
-    // A custom mode should be specifiable.
-    pub = pubRun(global: true, args: ["--mode", "custom-mode", "foo:script"]);
-    pub.stdout.expect("custom-mode");
-    pub.shouldExit();
-  });
-}