Add support for lazy transformers.

pkg/barback/lib/src/dry_run_transform.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.dry_run_transform;
+
+import 'asset_id.dart';
+import 'base_transform.dart';
+import 'transform_logger.dart';
+import 'transform_node.dart';
+
+/// A transform for [DryRunTransform]ers that allows them to declare the ids of
+/// the outputs they'll generate without generating the concrete bodies of those
+/// outputs.
+class DryRunTransform extends BaseTransform {

[Bob Nystrom, 2014/01/30 19:33:44]

Do we want to allow dry run transformers to read concrete inputs?

[nweiz, 2014/01/31 03:43:27]

I believe we do. We can optimize for the case where they don't actually consume
the contents of the inputs, but I think we should let them read the contents if
they want to.

+  final Set<AssetId> _outputIds;
+
+  DryRunTransform(TransformNode node, this._outputIds, LogFunction logFunction)
+    : super(node, logFunction);
+
+  /// Stores [id] as the id of an output created by this transformation.
+  ///
+  /// A transformation can output as many assets as it wants. If
+  /// [DryRunTransformer.dryRun] outputs a given asset id for a given input,
+  /// [Transformer.apply] should emit the corresponding asset as well.

[Bob Nystrom, 2014/01/30 19:33:44]

"should" -> "must".

What happens if it emits an asset that was *not* previously emitted by dry run?

[nweiz, 2014/01/31 03:43:27]

I don't think we should say "must", since we do handle this case pretty
gracefully. If they emit an undeclared output, that works fine. The only
behavioral degradation is that if the transform is lazy, accessing the
undeclared output won't trigger materialization (since we don't know that it
exists yet).

+  void addOutputId(AssetId id) {
+    // TODO(nweiz): This should immediately throw if an output with that ID
+    // has already been created by this transformer.
+    _outputIds.add(id);
+  }
+}