Use native async/await support in pub.

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

-// Copyright (c) 2013, 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 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import 'utils.dart';
-
-main() {
-  // This is a regression test for http://dartbug.com/16617.
-
-  initConfig();
-
-  integration(
-      "compiles dart.js and interop.js next to entrypoints when "
-          "browser is a dev dependency",
-      () {
-    // Dart2js can take a long time to compile dart code, so we increase the
-    // timeout to cope with that.
-    currentSchedule.timeout *= 3;
-
-    serveBrowserPackage();
-
-    d.dir(appPath, [d.pubspec({
-        "name": "myapp",
-        "dev_dependencies": {
-          "browser": "any"
-        }
-      }),
-          d.dir(
-              'web',
-              [d.file('file.dart', 'void main() => print("hello");')])]).create();
-
-    pubGet();
-
-    schedulePub(
-        args: ["build", "--all"],
-        output: new RegExp(r'Built 3 files to "build".'));
-
-    d.dir(
-        appPath,
-        [
-            d.dir(
-                'build',
-                [
-                    d.dir(
-                        'web',
-                        [
-                            d.dir(
-                                'packages',
-                                [
-                                    d.dir(
-                                        'browser',
-                                        [
-                                            d.file('dart.js', 'contents of dart.js'),
-                                            d.file('interop.js', 'contents of interop.js')])])])])]).validate();
-  });
-}