Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/hosted/fail_gracefully_on_missing_package_test.dart

-// Copyright (c) 2012, 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.
-
-library pub_tests;
-
-import '../../lib/src/exit_codes.dart' as exit_codes;
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-
-  forBothPubGetAndUpgrade((command) {
-    integration('fails gracefully if the package does not exist', () {
-      serveNoPackages();
-
-      d.appDir({
-        "foo": "1.2.3"
-      }).create();
-
-      pubCommand(command, error: new RegExp(r"""
-Could not find package foo at http://localhost:\d+\.
-Depended on by:
-- myapp""", multiLine: true), exitCode: exit_codes.UNAVAILABLE);
-    });
-  });
-}