Add support for lazy transformers.

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..6eed17af60f3b4695c0355556ede0e64ea2a8236 100644
--- a/pkg/barback/lib/src/transformer.dart
+++ b/pkg/barback/lib/src/transformer.dart
@@ -15,6 +15,9 @@ 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.
+///
+/// If possible, transformers should implement [DeclaringTransformer] as well to
+/// help barback optimize the package graph.
 abstract class Transformer {
   /// Override this to return a space-separated list of file extensions
   /// (with leading `.`) that are allowed for the primary inputs to this