Make Assets know their ID.

pkg/barback/lib/src/asset_node.dart

Index: pkg/barback/lib/src/asset_node.dart
diff --git a/pkg/barback/lib/src/asset_node.dart b/pkg/barback/lib/src/asset_node.dart
index 6fd86c622d7fab8058c23f134104e2f02690c719..7bef7a0afa1ad238b3607abbf28dd1f01fbf2252 100644
--- a/pkg/barback/lib/src/asset_node.dart
+++ b/pkg/barback/lib/src/asset_node.dart
@@ -17,17 +17,21 @@ import 'transform_node.dart';
 /// 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());
   }