Adding an aggregate transformer example to barback. Also updating the previous examples.

pkg/barback/example/simple_transformer/lib/transformer.dart

Index: pkg/barback/example/simple_transformer/lib/transformer.dart
diff --git a/pkg/barback/example/simple_transformer/lib/transformer.dart b/pkg/barback/example/simple_transformer/lib/transformer.dart
index 881e06fa5dceee63d8a5b47a68e2dd017b04bc1f..b005c2528f8ffaceeb9c0552a62331dc6f029562 100644
--- a/pkg/barback/example/simple_transformer/lib/transformer.dart
+++ b/pkg/barback/example/simple_transformer/lib/transformer.dart
@@ -14,8 +14,8 @@ class InsertCopyright extends Transformer {
   // class to be publicly available as a loadable transformer plugin.
   InsertCopyright.asPlugin();
 
-  Future<bool> isPrimary(Asset input) {
-    return new Future.value(input.id.extension == '.txt');
+  Future<bool> isPrimary(AssetId id) {
+    return new Future.value(id.extension == '.txt');
   }
 
   Future apply(Transform transform) {