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

sdk/lib/_internal/pub_generated/test/downgrade/doesnt_change_git_dependencies_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 '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("doesn't change git dependencies", () {
-    ensureGit();
-
-    d.git('foo.git', [d.libDir('foo'), d.libPubspec('foo', '1.0.0')]).create();
-
-    d.appDir({
-      "foo": {
-        "git": "../foo.git"
-      }
-    }).create();
-
-    pubGet();
-
-    d.dir(
-        packagesPath,
-        [d.dir('foo', [d.file('foo.dart', 'main() => "foo";')])]).validate();
-
-    d.git(
-        'foo.git',
-        [d.libDir('foo', 'foo 2'), d.libPubspec('foo', '1.0.0')]).commit();
-
-    pubDowngrade();
-
-    d.dir(
-        packagesPath,
-        [d.dir('foo', [d.file('foo.dart', 'main() => "foo";')])]).validate();
-  });
-}