Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/build/can_specify_output_directory_test.dart

-// Copyright (c) 2014, 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 'package:path/path.dart' as path;
-import 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("can specify the output directory to build into", () {
-    d.dir(
-        appPath,
-        [d.appPubspec(), d.dir('web', [d.file('file.txt', 'web')])]).create();
-
-    var outDir = path.join("out", "dir");
-    schedulePub(
-        args: ["build", "-o", outDir],
-        output: contains('Built 1 file to "$outDir".'));
-
-    d.dir(
-        appPath,
-        [
-            d.dir(
-                "out",
-                [d.dir("dir", [d.dir("web", [d.file("file.txt", "web")]),])])]).validate();
-  });
-}