Revert "Revert "Use native async/await support in pub.""

sdk/lib/_internal/pub_generated/test/get/hosted/unlock_if_version_doesnt_exist_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 'package:path/path.dart' as p;
-import 'package:scheduled_test/scheduled_test.dart';
-
-import '../../../lib/src/io.dart';
-import '../../descriptor.dart' as d;
-import '../../test_pub.dart';
-
-main() {
-  initConfig();
-  integration(
-      'upgrades a locked pub server package with a nonexistent version',
-      () {
-    servePackages((builder) => builder.serve("foo", "1.0.0"));
-
-    d.appDir({
-      "foo": "any"
-    }).create();
-    pubGet();
-    d.packagesDir({
-      "foo": "1.0.0"
-    }).validate();
-
-    schedule(() => deleteEntry(p.join(sandboxDir, cachePath)));
-
-    servePackages((builder) => builder.serve("foo", "1.0.1"), replace: true);
-    pubGet();
-    d.packagesDir({
-      "foo": "1.0.1"
-    }).validate();
-  });
-}