Add support for lazy transformers.

pkg/barback/lib/src/dry_run_transformer.dart

Index: pkg/barback/lib/src/dry_run_transformer.dart
diff --git a/pkg/barback/lib/src/dry_run_transformer.dart b/pkg/barback/lib/src/dry_run_transformer.dart
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+++ b/pkg/barback/lib/src/dry_run_transformer.dart
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+// 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.dry_run_transformer;
+
+import 'dart:async';
+
+import 'dry_run_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 DryRunTransformer {
+  /// Figure out 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 dryRun(DryRunTransform transform);
+}