Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/serve/supports_user_defined_declaring_transformers_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 '../descriptor.dart' as d;
-import '../test_pub.dart';
-import 'utils.dart';
-
-const DECLARING_TRANSFORMER = """
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-class DeclaringRewriteTransformer extends Transformer
-    implements DeclaringTransformer {
-  DeclaringRewriteTransformer.asPlugin();
-
-  String get allowedExtensions => '.out';
-
-  Future apply(Transform transform) {
-    transform.logger.info('Rewriting \${transform.primaryInput.id}.');
-    return transform.primaryInput.readAsString().then((contents) {
-      var id = transform.primaryInput.id.changeExtension(".final");
-      transform.addOutput(new Asset.fromString(id, "\$contents.final"));
-    });
-  }
-
-  Future declareOutputs(DeclaringTransform transform) {
-    transform.declareOutput(transform.primaryId.changeExtension(".final"));
-    return new Future.value();
-  }
-}
-""";
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("supports a user-defined declaring transformer", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/lazy", "myapp/src/declaring"]
-        }),
-            d.dir(
-                "lib",
-                [d.dir("src", [// Include a lazy transformer before the declaring transformer,
-            // because otherwise its behavior is indistinguishable from a normal
-            // transformer.
-            d.file("lazy.dart", LAZY_TRANSFORMER),
-                d.file("declaring.dart", DECLARING_TRANSFORMER)])]),
-                d.dir("web", [d.file("foo.txt", "foo")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      var server = pubServe();
-      // The build should complete without either transformer logging anything.
-      server.stdout.expect('Build completed successfully');
-
-      requestShouldSucceed("foo.final", "foo.out.final");
-      server.stdout.expect(
-          emitsLines(
-              '[Info from LazyRewrite]:\n' 'Rewriting myapp|web/foo.txt.\n'
-                  '[Info from DeclaringRewrite]:\n' 'Rewriting myapp|web/foo.out.'));
-      endPubServe();
-    });
-  });
-}