Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/get/path/path_is_file_test.dart

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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 d.file.
-
-import 'package:path/path.dart' as path;
-
-import '../../descriptor.dart' as d;
-import '../../test_pub.dart';
-
-main() {
-  initConfig();
-  integration('path dependency when path is a file', () {
-    d.dir('foo', [d.libDir('foo'), d.libPubspec('foo', '0.0.1')]).create();
-
-    d.file('dummy.txt', '').create();
-    var dummyPath = path.join(sandboxDir, 'dummy.txt');
-
-    d.dir(appPath, [d.appPubspec({
-        "foo": {
-          "path": dummyPath
-        }
-      })]).create();
-
-    pubGet(
-        error: 'Path dependency for package foo must refer to a '
-            'directory, not a file. Was "$dummyPath".');
-  });
-}