Fix a bug in barback caused by r33408.

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 'asset_cascade.dart';
 import 'asset_id.dart';
@@ skipping 307 matching lines @@
       // be re-run and will consume the output from the new final phase.
       output.removeListeners();
     }
     return _next;
   }
 
   /// Mark this phase as removed.
   ///
   /// This will remove all the phase's outputs and all following phases.
   void remove() {
-    _previous._next = null;
+    if (_previous != null) _previous._next = null;
     removeFollowing();
     for (var input in _inputs.values.toList()) {
       input.remove();
     }
     for (var group in _groups.values) {
       group.remove();
     }
     _onAssetController.close();
     _onLogPool.close();
   }
@@ skipping 59 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";
 }