Add support for lazy transformers.

pkg/barback/lib/src/declaring_transformer.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.declaring_transformer;
+
+import 'dart:async';
+
+import 'declaring_transform.dart';
+
+/// An interface for [Transformer]s that can cheaply figure out which assets
+/// they'll emit without doing the work of actually creating those assets.
+///
+/// If a transformer implements this interface, that allows barback to perform
+/// optimizations to make the asset graph work more smoothly.
+abstract class DeclaringTransformer {
+  /// Declare which assets would be emitted for the primary input specified by
+  /// [transform].
+  ///
+  /// For the most part, this works like [Transformer.apply]. The difference is
+  /// that instead of emitting [Asset]s, it just emits [AssetId]s through
+  /// [transform.addOutputId].
+  Future declareOutputs(DeclaringTransform transform);
+}