Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/cache/add/adds_latest_version_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.
-
-library pub_tests;
-
-import '../../descriptor.dart' as d;
-import '../../test_pub.dart';
-
-main() {
-  initConfig();
-  integration('adds the latest stable version of the package', () {
-    servePackages((builder) {
-      builder.serve("foo", "1.2.2");
-      builder.serve("foo", "1.2.3");
-      builder.serve("foo", "1.2.4-dev");
-    });
-
-    schedulePub(
-        args: ["cache", "add", "foo"],
-        output: 'Downloading foo 1.2.3...');
-
-    d.cacheDir({
-      "foo": "1.2.3"
-    }).validate();
-  });
-}