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

sdk/lib/_internal/pub_generated/asset/dart/serialize/get_input_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.get_input_transform;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:barback/barback.dart';
@@ skipping 14 matching lines @@
   Stream<List<int>> readInput(AssetId id) =>
       futureStream(getInput(id).then((input) => input.read()));
 
   Future<bool> hasInput(AssetId id) {
     return getInput(id).then((_) => true).catchError((error) {
       if (error is AssetNotFoundException && error.id == id) return false;
       throw error;
     });
   }
 }