Revert "Revert "Use native async/await support in pub.""

sdk/lib/_internal/pub_generated/test/transformer/exclusion/exclude_asset_string_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 '../../serve/utils.dart';
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("allows a single string as the asset to exclude", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": [{
-              "myapp/src/transformer": {
-                "\$exclude": "web/foo.txt"
-              }
-            }]
-        }),
-            d.dir("lib", [d.dir("src", [d.file("transformer.dart", REWRITE_TRANSFORMER)])]),
-            d.dir(
-                "web",
-                [
-                    d.file("foo.txt", "foo"),
-                    d.file("bar.txt", "bar"),
-                    d.dir("sub", [d.file("foo.txt", "foo"),])])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      pubServe();
-      requestShould404("foo.out");
-      requestShouldSucceed("sub/foo.out", "foo.out");
-      requestShouldSucceed("bar.out", "bar.out");
-      endPubServe();
-    });
-  });
-}