Fix a synchrony bug in barback.

pkg/barback/lib/src/group_runner.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.group_runner;
 
 import 'dart:async';
 
 import 'asset_cascade.dart';
 import 'asset_node.dart';
@@ skipping 67 matching lines @@
     // Process the first phase that needs to do work.
     for (var phase in _phases) {
       var future = phase.process();
       if (future != null) return future.then((_) => process());
     }
 
     // If we get here, all phases are done processing.
     var newOutputs = _phases.last.availableOutputs
         .difference(_alreadyEmittedOutputs);
     for (var output in newOutputs) {
-      output.whenRemoved.then((_) => _alreadyEmittedOutputs.remove(output));
+      output.whenRemoved(() => _alreadyEmittedOutputs.remove(output));
     }
     _alreadyEmittedOutputs.addAll(newOutputs);
 
     return new Future.value(newOutputs);
   }
 }