Make PhaseOutput emit a new AssetNode whenever it changes outputs.

pkg/barback/lib/src/phase.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;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset_cascade.dart';
@@ skipping 204 matching lines @@
   ///
   /// This may only be called on a phase with no phase following it.
   Phase addPhase(Iterable transformers) {
     assert(_next == null);
     _next = new Phase(cascade, transformers);
     for (var output in _outputs.values.toList()) {
       // Remove [output]'s listeners because now they should get the asset from
       // [_next], rather than this phase. Any transforms consuming [output] will
       // be re-run and will consume the output from the new final phase.
       output.removeListeners();
-
-      // Removing [output]'s listeners will cause it to be removed from
-      // [_outputs], so we have to put it back.
-      _outputs[output.output.id] = output;
-      output.output.whenRemoved.then((_) => _outputs.remove(output.output.id));
-      _next.addInput(output.output);
     }
     return _next;
   }
 
   /// Mark this phase as removed.
   ///
   /// This will remove all the phase's outputs and all following phases.
   void remove() {
     removeFollowing();
     for (var input in _inputs.values.toList()) {
@@ skipping 57 matching lines @@
       }
     });
   }
 
   /// Add [asset] as an output of this phase.
   void _addOutput(AssetNode asset) {
     if (_outputs.containsKey(asset.id)) {
       _outputs[asset.id].add(asset);
     } else {
       _outputs[asset.id] = new PhaseOutput(this, asset);
-      _outputs[asset.id].output.whenRemoved.then((_) {
-        _outputs.remove(asset.id);
-      });
+      _outputs[asset.id].onAsset.listen((output) {
+        if (_next != null) _next.addInput(output);
+      }, onDone: () => _outputs.remove(asset.id));
       if (_next != null) _next.addInput(_outputs[asset.id].output);
     }
   }
 }