Make barback's onAsset streams broadcast rather than single-subscriber.

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 'asset.dart';
 import 'asset_id.dart';
@@ skipping 164 matching lines @@
         continue;
       }
 
       var phase = _phases.last.addPhase();
       _addPhase(phase);
       phase.updateTransformers(transformers[i]);
     }
 
     if (transformers.length == 0) {
       _phases.last.updateTransformers([]);
-    } else if (transformers.length < _phases.length) {
-      _phases[transformers.length - 1].removeFollowing();
+    } else {
+      for (var i = transformers.length; i < _phases.length; i++) {
+        _phases[i].remove();
+      }
       _phases.removeRange(transformers.length, _phases.length);
     }
   }
 
   /// Force all [LazyTransformer]s' transforms in this cascade to begin
   /// producing concrete assets.
   void forceAllTransforms() {
     for (var phase in _phases) {
       phase.forceAllTransforms();
     }
   }
 
   void reportError(BarbackException error) {
     _errorsController.add(error);
   }
 
   /// Add [phase] to the end of [_phases] and watch its streams.
   void _addPhase(Phase phase) {
     _onLogPool.add(phase.onLog);
     phase.onDone.listen((_) {
       if (!isDirty) _onDoneController.add(null);
     });
 
     _phases.add(phase);
   }
 
   String toString() => "cascade for $package";
 }