Fix a few warnings.

pkg/barback/lib/src/phase_input.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_input;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
 import 'asset_forwarder.dart';
 import 'asset_node.dart';
 import 'errors.dart';
+import 'phase.dart';
 import 'stream_pool.dart';
 import 'transform_node.dart';
 import 'transformer.dart';
 import 'utils.dart';
 
 /// A class for watching a single [AssetNode] and running any transforms that
 /// take that node as a primary input.
 class PhaseInput {
   /// The phase for which this is an input.
   final Phase _phase;
@@ skipping 263 matching lines @@
       return new Future.value(
           new Set<AssetNode>.from([_passThroughController.node]));
     }
 
     return Future.wait(_transforms.map((transform) {
       if (!transform.isDirty) return new Future.value(new Set());
       return transform.apply();
     })).then((outputs) => unionAll(outputs));
   }
 }