Add a bunch of toString methods to internal Barback classes.

pkg/barback/lib/src/asset_set.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.asset_set;
 
 import 'dart:collection';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 50 matching lines @@
     return _assets.containsKey(id);
   }
 
   /// If the set contains an [Asset] with [id], removes and returns it.
   Asset removeId(AssetId id) => _assets.remove(id);
 
   /// Removes all assets from the set.
   void clear() {
     _assets.clear();
   }
+
+  String toString() => _assets.toString();
 }