Fix a bug in barback's static package handling.

pkg/barback/lib/src/graph/static_asset_cascade.dart

Index: pkg/barback/lib/src/graph/static_asset_cascade.dart
diff --git a/pkg/barback/lib/src/graph/static_asset_cascade.dart b/pkg/barback/lib/src/graph/static_asset_cascade.dart
index f909c0f703753db29c54bf829ca202a82b7d6823..5aab920cb7a9cfe797e6b34a62fcb1c13757a3ec 100644
--- a/pkg/barback/lib/src/graph/static_asset_cascade.dart
+++ b/pkg/barback/lib/src/graph/static_asset_cascade.dart
@@ -42,7 +42,7 @@ class StaticAssetCascade implements AssetCascade {
 
   Future<AssetSet> get availableOutputs {
     var provider = graph.provider as StaticPackageProvider;
-    return provider.getAllAssetIds(package).map(provider.getAsset).toList()
+    return provider.getAllAssetIds(package).asyncMap(provider.getAsset).toList()
         .then((assets) => new AssetSet.from(assets));
   }