Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/list_package_dirs/missing_pubspec_test.dart

Index: sdk/lib/_internal/pub_generated/test/list_package_dirs/missing_pubspec_test.dart
diff --git a/sdk/lib/_internal/pub_generated/test/list_package_dirs/missing_pubspec_test.dart b/sdk/lib/_internal/pub_generated/test/list_package_dirs/missing_pubspec_test.dart
deleted file mode 100644
index 0838cd2171b6585f37eedbc7ea5f76033baf3d43..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub_generated/test/list_package_dirs/missing_pubspec_test.dart
+++ /dev/null
@@ -1,23 +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 d.file.
-
-import 'package:path/path.dart' as path;
-
-import '../../lib/src/io.dart';
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  // This is a regression test for #20065.
-  integration("reports a missing pubspec error using JSON", () {
-    d.dir(appPath).create();
-
-    schedulePub(args: ["list-package-dirs", "--format=json"], outputJson: {
-      "error": 'Could not find a file named "pubspec.yaml" in "'
-          '${canonicalize(path.join(sandboxDir, appPath))}".',
-      "path": canonicalize(path.join(sandboxDir, appPath, "pubspec.yaml"))
-    }, exitCode: 1);
-  });
-}