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

sdk/lib/_internal/pub_generated/asset/dart/serialize/exception.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.exception;
 
 import 'package:barback/barback.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import '../utils.dart';
@@ skipping 82 matching lines @@
 ///
 /// This handles [AssetNotFoundException]s specially, ensuring that their
 /// metadata is preserved.
 CrossIsolateException deserializeException(Map error) {
   if (error['type'] == 'AssetNotFoundException') {
     return new _CrossIsolateAssetNotFoundException.deserialize(error);
   } else {
     return new CrossIsolateException.deserialize(error);
   }
 }