Add a bunch of toString methods to internal Barback classes.

pkg/barback/lib/src/asset_cascade.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_cascade;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 99 matching lines @@
 
   /// Returns all currently-available output assets from this cascade.
   AssetSet get availableOutputs =>
     new AssetSet.from(_phases.last.availableOutputs.map((node) => node.asset));
 
   /// Creates a new [AssetCascade].
   ///
   /// It loads source assets within [package] using [provider].
   AssetCascade(this.graph, this.package) {
     _onDirtyPool.add(_onDirtyController.stream);
-    _addPhase(new Phase(this, []));
+    _addPhase(new Phase(this, [], package));
 
     // Keep track of logged errors so we can know that the build failed.
     onLog.listen((entry) {
       if (entry.level == LogLevel.ERROR) {
         // TODO(nweiz): keep track of stack chain.
         _accumulatedErrors.add(
             new TransformerException(entry.transform, entry.message, null));
       }
     });
   }
@@ skipping 170 matching lines @@
         if (_newChanges) return _process();
 
         // Otherwise, everything is done.
         return null;
       }
 
       // Process that phase and then loop onto the next.
       return future.then((_) => _process());
     });
   }
+
+  String toString() => "cascade for $package";
 }