Make Assets know their ID.

pkg/barback/lib/src/asset_graph.dart

Index: pkg/barback/lib/src/asset_graph.dart
diff --git a/pkg/barback/lib/src/asset_graph.dart b/pkg/barback/lib/src/asset_graph.dart
index 5d77a5c8bc7944b24da693713285736f9c0454ad..125df9557e6a3f346fe6b2d5147eaad59addbabd 100644
--- a/pkg/barback/lib/src/asset_graph.dart
+++ b/pkg/barback/lib/src/asset_graph.dart
@@ -10,6 +10,7 @@ import 'dart:collection';
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_provider.dart';
+import 'asset_set.dart';
 import 'errors.dart';
 import 'change_batch.dart';
 import 'phase.dart';
@@ -190,12 +191,12 @@ class AssetGraph {
       var changes = _sourceChanges;
       _sourceChanges = null;
 
-      var updated = new Map<AssetId, Asset>();
+      var updated = new AssetSet();
       var futures = [];
       for (var id in changes.updated) {
         // TODO(rnystrom): Catch all errors from provider and route to results.
         futures.add(_provider.getAsset(id).then((asset) {
-          updated[id] = asset;
+          updated.add(asset);
         }).catchError((error) {
           if (error is AssetNotFoundException) {
             // Handle missing asset errors like regular missing assets.