Use shelf in pub's barback and admin servers.

sdk/lib/_internal/pub/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';
 
 import '../serialize.dart';
-import '../utils.dart';
 import 'transform.dart';
 
 /// Converts [transformer] into a serializable map.
 Map _serializeTransformer(Transformer transformer) {
   var port = new ReceivePort();
   port.listen((wrappedMessage) {
     respond(wrappedMessage, (message) {
       if (message['type'] == 'isPrimary') {
         return transformer.isPrimary(deserializeId(message['id']));
       } else if (message['type'] == 'declareOutputs') {
@@ skipping 42 matching lines @@
 
 /// Converts [transformerOrGroup] into a serializable map.
 Map serializeTransformerOrGroup(transformerOrGroup) {
   if (transformerOrGroup is Transformer) {
     return _serializeTransformer(transformerOrGroup);
   } else {
     assert(transformerOrGroup is TransformerGroup);
     return _serializeTransformerGroup(transformerOrGroup);
   }
 }