Fix type errors detected with --error-on-bad-type.

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';
@@ skipping 12 matching lines @@
 ///
 /// When the asset being forwarding changes, the old value of [output] will be
 /// marked as removed and a new value will replace it. Users of this class can
 /// be notified of this using [onAsset].
 class PhaseOutput {
   /// The phase for which this is an output.
   final Phase _phase;
 
   /// The asset node for this output.
   AssetNode get output => _outputForwarder.node;
-  AssetNodeForwarder _outputForwarder;
+  AssetForwarder _outputForwarder;
 
   /// A stream that emits an [AssetNode] each time this output starts forwarding
   /// a new asset.
   Stream<AssetNode> get onAsset => _onAssetController.stream;
   final _onAssetController = new StreamController<AssetNode>();
 
   /// The assets for this output.
   ///
   /// If there's no collision, this will only have one element. Otherwise, it
   /// will be ordered by which asset was added first.
@@ skipping 59 matching lines @@
       // 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));
       }
     });
   }
 }