Fix a few warnings.

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';
-import 'asset_id.dart';
 import 'asset_node.dart';
 import 'errors.dart';
-import 'phase_input.dart';
-import 'stream_pool.dart';
-import 'transformer.dart';
+import 'phase.dart';
 import 'utils.dart';
 
 /// A class that handles a single output of a phase.
 ///
 /// Normally there's only a single [AssetNode] for a phase's output, but it's
 /// possible that multiple transformers in the same phase emit assets with the
 /// same id, causing collisions. This handles those collisions by forwarding the
 /// chronologically first asset.
 class PhaseOutput {
   /// The phase for which this is an output.
@@ skipping 79 matching lines @@
     // If this was the first asset, we replace it with the next asset
     // (chronologically).
     if (wasFirst) {
       var newOutput = _assets.first;
       _outputController.setTransform(newOutput.transform);
       if (newOutput.state.isAvailable) {
         if (output.state.isAvailable) _outputController.setDirty();
         _outputController.setAvailable(newOutput.asset);
       } else {
         assert(newOutput.isDirty);
-        if (!output.isDirty) _outputController.setDirty();
+        if (!output.state.isDirty) _outputController.setDirty();
       }
     }
 
     // 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));
     }
   }
 }