Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/dart2js/passes_along_environment_constants_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 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../serve/utils.dart';
-
-main() {
-  initConfig();
-  integration("passes along environment constants", () {
-    d.dir(appPath, [d.pubspec({
-        "name": "myapp",
-        "transformers": [{
-            "\$dart2js": {
-              "environment": {
-                'CONSTANT': 'true'
-              }
-            }
-          }]
-      }), d.dir("web", [d.file("main.dart", """
-void main() {
-  if (const bool.fromEnvironment('CONSTANT')) {
-    print("hello");
-  }
-}
-""")])]).create();
-
-    pubServe();
-    requestShouldSucceed("main.dart.js", contains("hello"));
-    endPubServe();
-  });
-}