Fix a synchrony bug in barback.

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 64 matching lines @@
   /// a new [Phase] to cause consumers of the prior phase's outputs to be to
   /// start consuming the new phase's outputs instead.
   void removeListeners() {
     _outputForwarder.close();
     _outputForwarder = new AssetForwarder(_assets.first);
     _onAssetController.add(output);
   }
 
   /// Watches [node] to adjust [_assets] and [output] when it's removed.
   void _watchAsset(AssetNode node) {
-    node.whenRemoved.then((_) {
+    node.whenRemoved(() {
       if (_assets.length == 1) {
         assert(_assets.single == node);
         _outputForwarder.close();
         _onAssetController.close();
         return;
       }
 
       // If there was more than one asset, we're resolving a collision --
       // possibly partially.
       var wasFirst = _assets.first == node;
       _assets.remove(node);
 
       // If this was the first asset, we replace it with the next asset
       // (chronologically).
       if (wasFirst) removeListeners();
 
       // If there's still a collision, report it. This lets the user know if
       // they've successfully resolved the collision or not.
       if (_assets.length > 1) {
         // 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));
       }
     });
   }
 }