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

sdk/lib/_internal/pub_generated/asset/dart/serialize/transformer.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.transformer;
 
 import 'dart:async';
 import 'dart:isolate';
 
 import 'package:barback/barback.dart';
@@ skipping 106 matching lines @@
     return _serializeTransformer(transformerLike);
   } else if (transformerLike is TransformerGroup) {
     return _serializeTransformerGroup(transformerLike);
   } else {
     // This has to be last, since "transformerLike is AggregateTransformer" will
     // throw on older versions of barback.
     assert(transformerLike is AggregateTransformer);
     return _serializeAggregateTransformer(transformerLike);
   }
 }