Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/test/asset_set_test.dart

Index: third_party/pkg/barback-0.13.0/test/asset_set_test.dart
diff --git a/pkg/barback/test/asset_set_test.dart b/third_party/pkg/barback-0.13.0/test/asset_set_test.dart
similarity index 93%
copy from pkg/barback/test/asset_set_test.dart
copy to third_party/pkg/barback-0.13.0/test/asset_set_test.dart
index f339d7ef51f24f98065956de5a95211b0623fc81..3208329de03c18f81366268ec5955f4237a94bc3 100644
--- a/pkg/barback/test/asset_set_test.dart
+++ b/third_party/pkg/barback-0.13.0/test/asset_set_test.dart
@@ -146,14 +146,4 @@ 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]));
-    });
-  });
 }