Fix a synchrony bug in barback.

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 133 matching lines @@
     forwarder.onForwarding.listen((asset) {
       _addOutput(asset);
 
       var exception = _outputs[asset.id].collisionException;
       if (exception != null) cascade.reportError(exception);
     });
 
     _inputOrigins.add(node.origin);
     var input = new PhaseInput(this, node, _transformers);
     _inputs[node.id] = input;
-    input.input.whenRemoved.then((_) {
+    input.input.whenRemoved(() {
       _inputOrigins.remove(node.origin);
       _inputs.remove(node.id);
       _forwarders.remove(node.id).remove();
     });
     _onDirtyPool.add(input.onDirty);
     _onDirtyController.add(null);
     _onLogPool.add(input.onLog);
 
     for (var group in _groups.values) {
       group.addInput(node);
@@ skipping 143 matching lines @@
       _outputs[asset.id].add(asset);
     } else {
       _outputs[asset.id] = new PhaseOutput(this, asset);
       _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);
     }
   }
 }