Barback transforms now pass through the primary input by default.

pkg/barback/lib/src/phase_output.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.phase_output;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset_cascade.dart';
@@ skipping 20 matching lines @@
   /// A string describing the location of [this] in the transformer graph.
   final String _location;
 
   /// The asset node for this output.
   AssetNode get output => _outputForwarder.node;
   AssetForwarder _outputForwarder;
 
   /// A stream that emits an [AssetNode] each time this output starts forwarding
   /// a new asset.
   Stream<AssetNode> get onAsset => _onAssetController.stream;
-  final _onAssetController = new StreamController<AssetNode>();
+  final _onAssetController = new StreamController<AssetNode>(sync: true);
 
   /// The assets for this output.
   ///
   /// If there's no collision, this will only have one element. Otherwise, it
   /// will be ordered by which asset was added first.
   final _assets = new Queue<AssetNode>();
 
   /// The [AssetCollisionException] for this output, or null if there is no
   /// collision currently.
   AssetCollisionException get collisionException {
@@ skipping 56 matching lines @@
         // Pump the event queue to ensure that the removal of the input triggers
         // a new build to which we can attach the error.
         // TODO(nweiz): report this through the output asset.
         newFuture(() => _phase.cascade.reportError(collisionException));
       }
     });
   }
 
   String toString() => "phase output in $_location for $output";
 }