Add support for transformer clusters to barback.

pkg/barback/lib/src/transformer.dart

Index: pkg/barback/lib/src/transformer.dart
diff --git a/pkg/barback/lib/src/transformer.dart b/pkg/barback/lib/src/transformer.dart
index e8479f1f8143d2273593250e4aeab3bb48e7324d..07e107eb835c326458658d50904b1b188d130776 100644
--- a/pkg/barback/lib/src/transformer.dart
+++ b/pkg/barback/lib/src/transformer.dart
@@ -7,6 +7,7 @@ library barback.transformer;
 import 'dart:async';
 
 import 'asset.dart';
+import 'operator.dart';
 import 'transform.dart';
 
 /// A [Transformer] represents a processor that takes in one or more input
@@ -15,7 +16,7 @@ import 'transform.dart';
 /// Dart2js, a SASS->CSS processor, a CSS spriter, and a tool to concatenate
 /// files are all examples of transformers. To define your own transformation
 /// step, extend (or implement) this class.
-abstract class Transformer {
+abstract class Transformer implements Operator {
   /// Override this to return a space-separated list of file extensions
   /// (with leading `.`) that are allowed for the primary inputs to this
   /// transformer.