Add support for lazy transformers.

pkg/barback/lib/src/lazy_transformer.dart

Index: pkg/barback/lib/src/lazy_transformer.dart
diff --git a/pkg/barback/lib/src/lazy_transformer.dart b/pkg/barback/lib/src/lazy_transformer.dart
new file mode 100644
index 0000000000000000000000000000000000000000..67511f8ea4ad04beb44b6b51d6c2b6dc2860b60e
--- /dev/null
+++ b/pkg/barback/lib/src/lazy_transformer.dart
@@ -0,0 +1,16 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library barback.lazy_transformer;
+
+import 'declaring_transformer.dart';
+
+/// An interface for [Transformer]s that indicates that the transformer's
+/// outputs shouldn't be generated until requested.
+///
+/// The [declareOutputs] method is used to figure out which assets should be
+/// treated as "lazy." Lazy assets will only be forced to be generated if
+/// they're requested by the user or if they're used by a non-declaring
+/// transformer.
+abstract class LazyTransformer extends DeclaringTransformer {}