Transition barback's infrastructure to an aggregate-based model.

pkg/barback/test/asset_set_test.dart

Index: pkg/barback/test/asset_set_test.dart
diff --git a/pkg/barback/test/asset_set_test.dart b/pkg/barback/test/asset_set_test.dart
index 3208329de03c18f81366268ec5955f4237a94bc3..f339d7ef51f24f98065956de5a95211b0623fc81 100644
--- a/pkg/barback/test/asset_set_test.dart
+++ b/pkg/barback/test/asset_set_test.dart
@@ -146,4 +146,14 @@ main() {
       expect(set.removeId(barId), isNull);
     });
   });
+
+  group(".ids", () {
+    test("contains the ids of all the assets in the set", () {
+      var set = new AssetSet();
+      var foo = new Asset.fromString(fooId, "foo");
+      var bar = new Asset.fromString(barId, "bar");
+      set.addAll([foo, bar]);
+      expect(set.ids, unorderedEquals([fooId, barId]));
+    });
+  });
 }