Add chain support to barback.

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 104 matching lines @@
   /// 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, []));
 
     // 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 trace.
+        // TODO(nweiz): keep track of stack chain.
         _accumulatedErrors.add(
             new TransformerException(entry.transform, entry.message, null));
       }
     });
   }
 
   /// Gets the asset identified by [id].
   ///
   /// If [id] is for a generated or transformed asset, this will wait until it
   /// has been created and return it. This means that the returned asset will
@@ skipping 166 matching lines @@
 
         // Otherwise, everything is done.
         return null;
       }
 
       // Process that phase and then loop onto the next.
       return future.then((_) => _process());
     });
   }
 }