Make Assets know their ID.

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_graph.dart';
 import 'asset_id.dart';
 import 'phase.dart';
 import 'transform_node.dart';
 
 /// Describes an asset and its relationship to the build dependency graph.
 ///
 /// Keeps a cache of the last asset that was built for this node (i.e. for this
 /// node's ID and phase) and tracks which transforms depend on it.
 class AssetNode {
-  final AssetId id;
   Asset asset;
 
   /// The [TransformNode]s that consume this node's asset as an input.
   final consumers = new Set<TransformNode>();
 
-  AssetNode(this.id);
+  AssetId get id => asset.id;
+
+  AssetNode(this.asset);
 
   /// Updates this node's generated asset value and marks all transforms that
   /// use this as dirty.
   void updateAsset(Asset asset) {
+    // Cannot update an asset to one with a different ID.
+    assert(id == asset.id);
+
     this.asset = asset;
     consumers.forEach((consumer) => consumer.dirty());
   }
 }