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

sdk/lib/_internal/pub_generated/asset/dart/serialize/transform.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.transform;
 
 import 'dart:async';
 import 'dart:isolate';
 
 import 'package:barback/barback.dart';
@@ skipping 129 matching lines @@
       : primaryId = deserializeId(transform['primaryId']),
         super(transform);
 
   void declareOutput(AssetId id) {
     call(_port, {
       'type': 'declareOutput',
       'output': serializeId(id)
     });
   }
 }