Add EfficientLength marker interface to some iterabels.

pkg/barback/lib/src/asset_set.dart

Index: pkg/barback/lib/src/asset_set.dart
diff --git a/pkg/barback/lib/src/asset_set.dart b/pkg/barback/lib/src/asset_set.dart
index 08c2db9b7bfd985ff445dd6443b6c669d31f10be..66601b9366bfee7cfd271b4ca000701e40d566e3 100644
--- a/pkg/barback/lib/src/asset_set.dart
+++ b/pkg/barback/lib/src/asset_set.dart
@@ -13,7 +13,7 @@ import 'asset_id.dart';
 ///
 /// This uses the [AssetId] of each asset to determine uniqueness, so no two
 /// assets with the same ID can be in the set.
-class AssetSet extends IterableBase<Asset> {
+class AssetSet extends IterableBase<Asset> implements EfficientLength {
   final _assets = new Map<AssetId, Asset>();
 
   AssetSet();