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

sdk/lib/_internal/pub_generated/test/downgrade/unlock_if_necessary_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(
-      "downgrades one locked hosted package's dependencies if it's " "necessary",
-      () {
-    servePackages((builder) {
-      builder.serve("foo", "2.0.0", deps: {
-        "foo_dep": "any"
-      });
-      builder.serve("foo_dep", "2.0.0");
-    });
-
-    d.appDir({
-      "foo": "any"
-    }).create();
-
-    pubGet();
-
-    d.packagesDir({
-      "foo": "2.0.0",
-      "foo_dep": "2.0.0"
-    }).validate();
-
-    servePackages((builder) {
-      builder.serve("foo", "1.0.0", deps: {
-        "foo_dep": "<2.0.0"
-      });
-      builder.serve("foo_dep", "1.0.0");
-    });
-
-    pubDowngrade(args: ['foo']);
-
-    d.packagesDir({
-      "foo": "1.0.0",
-      "foo_dep": "1.0.0"
-    }).validate();
-  });
-}