Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/build/outputs_error_to_json_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:scheduled_test/scheduled_test.dart';
-
-import '../../lib/src/exit_codes.dart' as exit_codes;
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-const TRANSFORMER = """
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-class RewriteTransformer extends Transformer {
-  RewriteTransformer.asPlugin();
-
-  String get allowedExtensions => '.txt';
-
-  Future apply(Transform transform) => throw new Exception('oh no!');
-}
-""";
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("outputs error to JSON in a failed build", () {
-      // Loading transformers takes several seconds, so make sure we don't
-      // timeout.
-      currentSchedule.timeout *= 2;
-
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp"]
-        }),
-            d.dir("lib", [d.file("transformer.dart", TRANSFORMER)]),
-            d.dir("web", [d.file("foo.txt", "foo")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      schedulePub(args: ["build", "--format", "json"], outputJson: {
-        "buildResult": "failure",
-        "errors": [{
-            "error": startsWith(
-                "Transform Rewrite on myapp|web/foo.txt " "threw error: oh no!")
-          }],
-        "log": []
-      }, exitCode: exit_codes.DATA);
-    });
-  });
-}