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

sdk/lib/_internal/pub_generated/asset/dart/utils.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.
 
 /// Functions go in this file as opposed to lib/src/utils.dart if they need to
 /// be accessible to the transformer-loading isolate.
 library pub.asset.utils;
 
 import 'dart:async';
 
@@ skipping 66 matching lines @@
     subscription = callback().listen(controller.add,
         onError: controller.addError,
         onDone: controller.close);
   },
       onCancel: () => subscription.cancel(),
       onPause: () => subscription.pause(),
       onResume: () => subscription.resume(),
       sync: true);
   return controller.stream;
 }