Fix a bug that caused the fix in r36391 not to work.

pkg/barback/lib/src/graph/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.graph.phase;
 
 import 'dart:async';
 
 import '../asset/asset_id.dart';
 import '../asset/asset_node.dart';
@@ skipping 69 matching lines @@
   /// The streams exposed by this phase.
   final _streams = new NodeStreams();
   Stream<NodeStatus> get onStatusChange => _streams.onStatusChange;
   Stream<AssetNode> get onAsset => _streams.onAsset;
   Stream<LogEntry> get onLog => _streams.onLog;
 
   /// How far along [this] is in processing its assets.
   NodeStatus get status {
     // Before any transformers are added, the phase should be dirty if and only
     // if any input is dirty.
-    if (_classifiers.isEmpty && _groups.isEmpty) {
+    if (_classifiers.isEmpty && _groups.isEmpty && previous == null) {
       return _inputs.any((input) => input.state.isDirty) ?
           NodeStatus.RUNNING : NodeStatus.IDLE;
     }
 
     var classifierStatus = NodeStatus.dirtiest(
         _classifiers.values.map((classifier) => classifier.status));
     var groupStatus = NodeStatus.dirtiest(
         _groups.values.map((group) => group.status));
     return (previous == null ? NodeStatus.IDLE : previous.status)
         .dirtier(classifierStatus)
@@ skipping 277 matching lines @@
     assert(asset.state.isDirty);
     asset.force();
     asset.whenStateChanges().then((state) {
       if (state.isRemoved) return getOutput(asset.id);
       return asset;
     }).then(request.complete).catchError(request.completeError);
   }
 
   String toString() => "phase $_location.$_index";
 }