Add chain support to barback.

pkg/barback/lib/src/internal_asset.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 barback.internal_asset;
 
 import 'dart:async';
 import 'dart:convert';
-import 'dart:io';
 import 'dart:typed_data';
 
 import 'asset.dart';
 import 'asset_id.dart';
 import 'file_pool.dart';
 import 'serialize.dart';
 import 'stream_replayer.dart';
 import 'utils.dart';
 
 /// Serialize an asset to a form that's safe to send across isolates.
@@ skipping 159 matching lines @@
   Future<String> readAsString({Encoding encoding}) {
     if (encoding == null) encoding = UTF8;
     return _replayer.getReplay().toList()
         .then((chunks) => encoding.decode(flatten(chunks)));
   }
 
   Stream<List<int>> read() => _replayer.getReplay();
 
   String toString() => "Stream";
 }