Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/serve/web_socket/path_to_urls_with_line_test.dart

-// Copyright (c) 2014, 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 file.
-
-library pub_tests;
-
-import 'package:path/path.dart' as p;
-
-import '../../descriptor.dart' as d;
-import '../../test_pub.dart';
-import '../utils.dart';
-
-main() {
-  initConfig();
-  integration("pathToUrls provides output line if given source", () {
-    d.dir(
-        appPath,
-        [d.appPubspec(), d.dir("web", [d.file("main.dart", "main"),])]).create();
-
-    pubServe();
-
-    expectWebSocketResult("pathToUrls", {
-      "path": p.join("web", "main.dart"),
-      "line": 12345
-    }, {
-      "urls": [getServerUrl("web", "main.dart")],
-      "line": 12345
-    });
-
-    endPubServe();
-  });
-}