Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/serve/does_not_serve_dart_in_release_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 'utils.dart';
-
-main() {
-  initConfig();
-  integration("does not serve .dart files in release mode", () {
-
-    d.dir(
-        "foo",
-        [d.libPubspec("foo", "0.0.1"), d.dir("lib", [d.file("foo.dart", """
-            library foo;
-            foo() => 'foo';
-            """)])]).create();
-
-    d.dir(appPath, [d.appPubspec({
-        "foo": {
-          "path": "../foo"
-        }
-      }),
-          d.dir("lib", [d.file("lib.dart", "lib() => print('hello');"),]),
-          d.dir("web", [d.file("file.dart", """
-            import 'package:foo/foo.dart';
-            main() => print('hello');
-            """),
-            d.dir("sub", [d.file("sub.dart", "main() => 'foo';"),])])]).create();
-
-    pubServe(shouldGetFirst: true, args: ["--mode", "release"]);
-    requestShould404("file.dart");
-    requestShould404("packages/myapp/lib.dart");
-    requestShould404("packages/foo/foo.dart");
-    requestShould404("sub/sub.dart");
-    endPubServe();
-  });
-}