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

sdk/lib/_internal/pub_generated/test/pub_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS 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 'package:scheduled_test/scheduled_test.dart';
 
 import '../lib/src/exit_codes.dart' as exit_codes;
 import 'test_pub.dart';
@@ skipping 37 matching lines @@
 
         Run "pub help <command>" for more information about a command.
         See http://dartlang.org/tools/pub for detailed documentation.
         """);
   });
 
   integration('running pub with just --version displays version', () {
     schedulePub(args: ['--version'], output: 'Pub 0.1.2+3');
   });
 }