Better handling of asset collisions in barback.

pkg/barback/lib/src/transform_node.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.transform_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 80 matching lines @@
       controller.setDirty();
     }
     _onDirtyController.add(null);
   }
 
   /// Applies this transform.
   ///
   /// Returns a set of asset nodes representing the outputs from this transform
   /// that weren't emitted last time it was run.
   Future<Set<AssetNode>> apply() {
+    assert(!_onDirtyController.isClosed);
+
     var newOutputs = new AssetSet();
     var transform = createTransform(this, newOutputs);
 
     // Clear all the old input subscriptions. If an input is re-used, we'll
     // re-subscribe.
     for (var subscription in _inputSubscriptions.values) {
       subscription.cancel();
     }
     _inputSubscriptions.clear();
 
@@ skipping 73 matching lines @@
       } else {
         var controller = new AssetNodeController.available(asset);
         _outputControllers[asset.id] = controller;
         brandNewOutputs.add(controller.node);
       }
     }
 
     return brandNewOutputs;
   }
 }