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

sdk/lib/_internal/pub_generated/lib/src/barback/dart_forwarding_transformer.dart

 // Copyright (c) 2013, 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.dart_forwarding_transformer;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
@@ skipping 12 matching lines @@
   DartForwardingTransformer(this._mode);
 
   String get allowedExtensions => ".dart";
 
   Future apply(Transform transform) {
     return newFuture(() {
       transform.addOutput(transform.primaryInput);
     });
   }
 }