Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/lib/src/asset_set.dart

Index: third_party/pkg/barback-0.13.0/lib/src/asset_set.dart
diff --git a/pkg/barback/lib/src/asset/asset_set.dart b/third_party/pkg/barback-0.13.0/lib/src/asset_set.dart
similarity index 94%
copy from pkg/barback/lib/src/asset/asset_set.dart
copy to third_party/pkg/barback-0.13.0/lib/src/asset_set.dart
index ac58774cafd221ccc1fb31aa2712fae606164ce9..b17db97491452262bc0016509bee039788c775a7 100644
--- a/pkg/barback/lib/src/asset/asset_set.dart
+++ b/third_party/pkg/barback-0.13.0/lib/src/asset_set.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-library barback.asset.asset_set;
+library barback.asset_set;
 
 import 'dart:collection';
 
@@ -16,9 +16,6 @@ import 'asset_id.dart';
 class AssetSet extends IterableBase<Asset> {
   final _assets = new Map<AssetId, Asset>();
 
-  /// The ids of the assets in the set.
-  Iterable<AssetId> get ids => _assets.keys;
-
   AssetSet();
 
   /// Creates a new AssetSet from the contents of [other].