Move common streams in barback to their own class.

pkg/barback/lib/src/group_runner.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.group_runner;
 
 import 'dart:async';
 
 import 'asset_cascade.dart';
 import 'asset_node.dart';
@@ skipping 69 matching lines @@
     }
   }
 
   /// Adds a new asset as an input for this group.
   void addInput(AssetNode node) {
     _phases.first.addInput(node);
   }
 
   /// Removes this group and all sub-phases within it.
   void remove() {
+    _onLogPool.close();
     for (var phase in _phases) {
       phase.remove();
     }
   }
 
   String toString() => "group in phase $_location for $_group";
 }