Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/get/git/check_out_branch_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 '../../descriptor.dart' as d;
-import '../../test_pub.dart';
-
-main() {
-  initConfig();
-  integration('checks out a package at a specific branch from Git', () {
-    ensureGit();
-
-    var repo =
-        d.git('foo.git', [d.libDir('foo', 'foo 1'), d.libPubspec('foo', '1.0.0')]);
-    repo.create();
-    repo.runGit(["branch", "old"]);
-
-    d.git(
-        'foo.git',
-        [d.libDir('foo', 'foo 2'), d.libPubspec('foo', '1.0.0')]).commit();
-
-    d.appDir({
-      "foo": {
-        "git": {
-          "url": "../foo.git",
-          "ref": "old"
-        }
-      }
-    }).create();
-
-    pubGet();
-
-    d.dir(
-        packagesPath,
-        [d.dir('foo', [d.file('foo.dart', 'main() => "foo 1";')])]).validate();
-  });
-}