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

sdk/lib/_internal/pub_generated/test/list_package_dirs/missing_pubspec_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 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 "'
-                '${canonicalize(path.join(sandboxDir, appPath))}".',
-          "path": canonicalize(path.join(sandboxDir, appPath, "pubspec.yaml"))
-        },
-        exitCode: 1);
+    schedulePub(args: ["list-package-dirs", "--format=json"], outputJson: {
+      "error": 'Could not find a file named "pubspec.yaml" in "'
+          '${canonicalize(path.join(sandboxDir, appPath))}".',
+      "path": canonicalize(path.join(sandboxDir, appPath, "pubspec.yaml"))
+    }, exitCode: 1);
   });
-}
+}