Add support for lazy transformers.

pkg/barback/test/transformer/lazy_rewrite.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_rewrite;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+import 'rewrite.dart';
+
+/// Like [RewriteTransformer], but returns a lazy asset that doesn't perform the
+/// rewrite until it's materialized.
+class LazyRewriteTransformer extends RewriteTransformer
+    implements LazyTransformer {
+  LazyRewriteTransformer(String from, String to)
+      : super(from, to);
+
+  Future declareOutputs(DeclaringTransform transform) {
+    for (var extension in to.split(" ")) {
+      var id = transform.primaryInput.id.changeExtension(".$extension");
+      transform.declareOutput(id);
+    }
+  }
+}