Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/serve/gets_first_if_source_changed_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 file.
-
-library pub_tests;
-
-import 'dart:convert';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import 'utils.dart';
-
-main() {
-  initConfig();
-  integration(
-      "gets first if a dependency's source doesn't match the one in " "the lock file",
-      () {
-    d.dir("foo", [d.libPubspec("foo", "0.0.1"), d.libDir("foo")]).create();
-
-    d.dir(appPath, [// A pubspec with a path source.
-      d.appPubspec({
-        "foo": {
-          "path": "../foo"
-        }
-      }), // A lock file with the hosted source.
-      d.file("pubspec.lock", JSON.encode({
-        'packages': {
-          'foo': {
-            'version': '0.0.0',
-            'source': 'hosted',
-            'description': 'foo'
-          }
-        }
-      }))]).create();
-
-    pubServe(shouldGetFirst: true);
-    requestShouldSucceed("packages/foo/foo.dart", 'main() => "foo";');
-    endPubServe();
-  });
-}