Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/serve/missing_asset_test.dart

Index: sdk/lib/_internal/pub_generated/test/serve/missing_asset_test.dart
diff --git a/sdk/lib/_internal/pub_generated/test/serve/missing_asset_test.dart b/sdk/lib/_internal/pub_generated/test/serve/missing_asset_test.dart
deleted file mode 100644
index 8bb69b0f4e429a523b8f932331b861def6d5b878..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub_generated/test/serve/missing_asset_test.dart
+++ /dev/null
@@ -1,22 +0,0 @@
-// 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("responds with a 404 for missing assets", () {
-    d.dir(appPath, [d.appPubspec()]).create();
-
-    pubServe();
-    requestShould404("index.html");
-    requestShould404("packages/myapp/nope.dart");
-    requestShould404("dir/packages/myapp/nope.dart");
-    endPubServe();
-  });
-}