Make PhaseOutput emit a new AssetNode whenever it changes outputs.

pkg/barback/lib/src/asset_node.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.asset_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 174 matching lines @@
   /// It's an error to mark an already-available node as available. It should be
   /// marked as dirty first.
   void setAvailable(Asset asset) {
     assert(asset.id == node.id);
     assert(node._state != AssetState.REMOVED);
     assert(node._state != AssetState.AVAILABLE);
     node._state = AssetState.AVAILABLE;
     node._asset = asset;
     node._stateChangeController.add(AssetState.AVAILABLE);
   }
-
-  /// Sets the origin of [node] to [origin] and the transform to
-  /// `origin.transform`.
-  void setOrigin(AssetNode origin) {
-    node._origin = origin;
-    node._transform = origin.transform;
-  }
 }
 
 // TODO(nweiz): add an error state.
 /// An enum of states that an [AssetNode] can be in.
 class AssetState {
   /// The node has a concrete asset loaded, available, and up-to-date. The asset
   /// is accessible via [AssetNode.asset]. An asset can only be marked available
   /// again from the [AssetState.DIRTY] state.
   static final AVAILABLE = const AssetState._("available");
 
@@ skipping 13 matching lines @@
 
   /// Whether this state is [AssetState.DIRTY].
   bool get isDirty => this == AssetState.DIRTY;
 
   final String name;
 
   const AssetState._(this.name);
 
   String toString() => name;
 }