Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/transformer/prints_a_transform_interface_error_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 '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../serve/utils.dart';
-
-final transformer = """
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-class RewriteTransformer extends Transformer {
-  RewriteTransformer.asPlugin();
-
-  String get allowedExtensions => '.txt';
-}
-""";
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("prints a transform interface error", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
-        }),
-            d.dir("lib", [d.dir("src", [d.file("transformer.dart", transformer)])]),
-            d.dir("web", [d.file("foo.txt", "foo")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      var server = pubServe();
-      server.stderr.expect(
-          emitsLines(
-              "Build error:\n" "Transform Rewrite on myapp|web/foo.txt threw error: Class "
-                  "'RewriteTransformer' has no instance method 'apply'."));
-      endPubServe();
-    });
-  });
-}