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

sdk/lib/_internal/pub_generated/test/get/path/nonexistent_dir_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 '../../../lib/src/exit_codes.dart' as exit_codes;
-import '../../descriptor.dart' as d;
-import '../../test_pub.dart';
-
-main() {
-  initConfig();
-  integration('path dependency to non-existent directory', () {
-    var badPath = path.join(sandboxDir, "bad_path");
-
-    d.dir(appPath, [d.appPubspec({
-        "foo": {
-          "path": badPath
-        }
-      })]).create();
-
-    pubGet(error: """
-        Could not find package foo at "$badPath".
-        Depended on by:
-        - myapp 0.0.0""", exitCode: exit_codes.UNAVAILABLE);
-  });
-}