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

sdk/lib/_internal/pub_generated/asset/dart/serialize.dart

 // Copyright (c) 2014, 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.asset.serialize;
 
 import 'dart:async';
 import 'dart:isolate';
 
 import 'package:barback/barback.dart';
@@ skipping 164 matching lines @@
   var replyTo = wrappedMessage['replyTo'];
   new Future.sync(() => callback(wrappedMessage['message']))
       .then((result) => replyTo.send({'type': 'success', 'value': result}))
       .catchError((error, stackTrace) {
     replyTo.send({
       'type': 'error',
       'error': serializeException(error, stackTrace)
     });
   });
 }