Add support for lazy transformers.

pkg/barback/test/transformer/lazy_many_to_one.dart

+// 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.test.transformer.lazy_many_to_one;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+import 'many_to_one.dart';
+
+/// Like [ManyToOneTransformer], but returns a lazy asset that doesn't perform
+/// the conglomeration until it's materialized.
+class LazyManyToOneTransformer extends ManyToOneTransformer
+    implements LazyTransformer {
+  LazyManyToOneTransformer(String extension)
+      : super(extension);
+
+  Future declareOutputs(DeclaringTransform transform) {
+    transform.declareOutput(transform.primaryInput.id.changeExtension(".out"));
+  }
+}