Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/get/path/absolute_symlink_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(
-      "generates a symlink with an absolute path if the dependency "
-          "path was absolute",
-      () {
-    d.dir("foo", [d.libDir("foo"), d.libPubspec("foo", "0.0.1")]).create();
-
-    d.dir(appPath, [d.appPubspec({
-        "foo": {
-          "path": path.join(sandboxDir, "foo")
-        }
-      })]).create();
-
-    pubGet();
-
-    d.dir("moved").create();
-
-    // Move the app but not the package. Since the symlink is absolute, it
-    // should still be able to find it.
-    scheduleRename(appPath, path.join("moved", appPath));
-
-    d.dir(
-        "moved",
-        [
-            d.dir(
-                packagesPath,
-                [d.dir("foo", [d.file("foo.dart", 'main() => "foo";')])])]).validate();
-  });
-}