Add support for lazy transformers.

pkg/barback/lib/src/declaring_transform.dart

Index: pkg/barback/lib/src/declaring_transform.dart
diff --git a/pkg/barback/lib/src/declaring_transform.dart b/pkg/barback/lib/src/declaring_transform.dart
new file mode 100644
index 0000000000000000000000000000000000000000..3e621d9b3519abb97de482c75a1aa72c3f765122
--- /dev/null
+++ b/pkg/barback/lib/src/declaring_transform.dart
@@ -0,0 +1,34 @@
+// 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.declaring_transform;
+
+import 'asset_id.dart';
+import 'base_transform.dart';
+import 'transform_logger.dart';
+import 'transform_node.dart';
+
+/// A transform for [DeclaringTransform]ers that allows them to declare the ids
+/// of the outputs they'll generate without generating the concrete bodies of
+/// those outputs.
+class DeclaringTransform extends BaseTransform {
+  final Set<AssetId> _outputIds;
+
+  DeclaringTransform(TransformNode node, this._outputIds,
+      LogFunction logFunction)
+    : super(node, logFunction);
+
+  /// Stores [id] as the id of an output that will be created by this
+  /// transformation when it's run.
+  ///
+  /// A transformation can declare as many assets as it wants. If
+  /// [DeclaringTransformer.declareOutputs] declareds a given asset id for a
+  /// given input, [Transformer.apply] should emit the corresponding asset as
+  /// well.
+  void declareOutput(AssetId id) {
+    // TODO(nweiz): This should immediately throw if an output with that ID
+    // has already been declared by this transformer.
+    _outputIds.add(id);
+  }
+}