Reorganize barback's source files.

pkg/barback/lib/src/utils/file_pool.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.file_pool;
+library barback.utils.file_pool;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:stack_trace/stack_trace.dart';
 
+import '../utils.dart';
 import 'pool.dart';
-import 'utils.dart';
 
 /// Manages a pool of files that are opened for reading to cope with maximum
 /// file descriptor limits.
 ///
 /// If a file cannot be opened because too many files are already open, this
 /// will defer the open until a previously opened file is closed and then try
 /// again. If this doesn't succeed after a certain amount of time, the open
 /// will fail and the original "too many files" exception will be thrown.
 class FilePool {
   /// The underlying pool.
@@ skipping 32 matching lines @@
     var completer = new Completer<List<int>>();
     var builder = new BytesBuilder();
 
     openRead(path).listen(builder.add, onDone: () {
       completer.complete(builder.takeBytes());
     }, onError: completer.completeError, cancelOnError: true);
 
     return completer.future;
   }
 }