Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/run/runs_the_script_in_checked_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 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-const SCRIPT = """
-main() {
-  int a = true;
-}
-""";
-
-main() {
-  initConfig();
-  integration('runs the script in checked mode by default', () {
-    d.dir(
-        appPath,
-        [d.appPubspec(), d.dir("bin", [d.file("script.dart", SCRIPT)])]).create();
-
-    schedulePub(
-        args: ["run", "script"],
-        error: contains("'bool' is not a subtype of type 'int' of 'a'"),
-        exitCode: 255);
-  });
-}