Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/run/includes_parent_directories_of_entrypoint_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:path/path.dart' as path;
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-const SCRIPT = r"""
-import '../../a.dart';
-import '../b.dart';
-main() {
-  print("$a $b");
-}
-""";
-
-main() {
-  initConfig();
-  integration(
-      'allows assets in parent directories of the entrypoint to be' 'accessed',
-      () {
-    d.dir(
-        appPath,
-        [
-            d.appPubspec(),
-            d.dir(
-                "tool",
-                [
-                    d.file("a.dart", "var a = 'a';"),
-                    d.dir(
-                        "a",
-                        [
-                            d.file("b.dart", "var b = 'b';"),
-                            d.dir("b", [d.file("app.dart", SCRIPT)])])])]).create();
-
-    var pub = pubRun(args: [path.join("tool", "a", "b", "app")]);
-    pub.stdout.expect("a b");
-    pub.shouldExit();
-  });
-}