Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/dart2js/minify_configuration_overrides_mode_test.dart

-// Copyright (c) 2013, 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.
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../serve/utils.dart';
-
-main() {
-  initConfig();
-  integration("minify configuration overrides the mode", () {
-    d.dir(appPath, [d.pubspec({
-        "name": "myapp",
-        "transformers": [{
-            "\$dart2js": {
-              "minify": true
-            }
-          }]
-      }),
-          d.dir(
-              "web",
-              [d.file("main.dart", "void main() => print('Hello!');")])]).create();
-
-    pubServe();
-    requestShouldSucceed("main.dart.js", isMinifiedDart2JSOutput);
-    endPubServe();
-  });
-}