Reorganize barback's source files.

pkg/barback/lib/src/utils/stream_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.stream_pool;
+library barback.utils.stream_pool;
 
 import 'dart:async';
 
 /// A pool of streams whose events are unified and emitted through a central
 /// stream.
 class StreamPool<T> {
   /// The stream through which all events from streams in the pool are emitted.
   Stream<T> get stream => _controller.stream;
   final StreamController<T> _controller;
 
@@ skipping 41 matching lines @@
 
   /// Removes all streams from this pool and closes [stream].
   void close() {
     for (var subscription in _subscriptions.values) {
       subscription.cancel();
     }
     _subscriptions.clear();
     _controller.close();
   }
 }