Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/global/binstubs/missing_script_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 p;
-
-import '../../descriptor.dart' as d;
-import '../../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("errors if an executable's script can't be found", () {
-    d.dir("foo", [d.pubspec({
-        "name": "foo",
-        "executables": {
-          "missing": "not_here",
-          "nope": null
-        }
-      })]).create();
-
-    var pub = startPub(args: ["global", "activate", "-spath", "../foo"]);
-
-    pub.stderr.expect(
-        'Warning: Executable "missing" runs '
-            '"${p.join('bin', 'not_here.dart')}", which was not found in foo.');
-    pub.stderr.expect(
-        'Warning: Executable "nope" runs '
-            '"${p.join('bin', 'nope.dart')}", which was not found in foo.');
-    pub.shouldExit();
-  });
-}