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..488c284b7bcabd532854a3111ec3e8cba556621a 100644
--- a/pkg/barback/lib/src/asset_graph.dart
+++ b/pkg/barback/lib/src/asset_graph.dart
@@ -190,12 +190,12 @@ class AssetGraph {
       var changes = _sourceChanges;
       _sourceChanges = null;
 
-      var updated = new Map<AssetId, Asset>();
+      var updated = new Set<Asset>();

[nweiz, 2013/07/03 20:08:25]

Does Set<Asset> work? Asset doesn't define [hash] or [operator==]. It would also
be weird to define those as forwarding to [AssetId], because then different
assets with the same id would register as equal.

It may be worth making a custom AssetSet class that wraps a Map<AssetId, Asset>.
That's a pattern that comes up a lot in the code, and it would be nice to have a
class that explicitly expresses "this is a set that's expected to contain only
one asset of a given id" without making the equality semantics of Asset weird.

[Bob Nystrom, 2013/07/03 22:32:11]

Object implements hashCode and == (based on identity), so all objects can be put
in a set. Asset could generate a more appropriate/faster hashCode based on the
ID, but it should work regardless.
If you implement ==, you must implement hashCode, but the converse isn't
necessarily true. It's OK to have your own optimized hashCode but still fallback
on identity for equality.
Great idea. Done.

       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.