Make Assets know their ID.

pkg/barback/lib/src/transform.dart

 // Copyright (c) 2013, 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.transform;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
+import 'asset_set.dart';
 import 'errors.dart';
 import 'transform_node.dart';
 
 /// Creates a [Transform] by forwarding to the private constructor.
 ///
 /// Lets [TransformNode] create [Transforms] without giving a [Transform]
 /// itself a public constructor, which would be visible to external users.
 /// Unlike the [Transform] class, this function is not exported by barback.dart.
 Transform createTransform(TransformNode node, Set<AssetNode> inputs,
-                          Map<AssetId, Asset> outputs) =>
+                          Set<Asset> outputs) =>
     new Transform._(node, inputs, outputs);
 
 /// While a [Transformer] represents a *kind* of transformation, this defines
 /// one specific usage of it on a set of files.
 ///
 /// This ephemeral object exists only during an actual transform application to
 /// facilitate communication between the [Transformer] and the code hosting
 /// the transformation. It lets the [Transformer] access inputs and generate
 /// outputs.
 class Transform {
   final TransformNode _node;
 
   final Set<AssetNode> _inputs;
-  final Map<AssetId, Asset> _outputs;
+  final AssetSet _outputs;
 
   /// Gets the ID of the primary input for this transformation.
   ///
   /// While a transformation can use multiple input assets, one must be a
   /// special "primary" asset. This will be the "entrypoint" or "main" input
   /// file for a transformation.
   ///
   /// For example, with a dart2js transform, the primary input would be the
   /// entrypoint Dart file. All of the other Dart files that that imports
   /// would be secondary inputs.
@@ skipping 18 matching lines @@
       // is exited. We'll then catch this and report it through the proper
       // results stream.
       if (node == null) throw new MissingInputException(id);
 
       // Keep track of which assets this transform depends on.
       _inputs.add(node);
       return node.asset;
     });
   }
 
-  /// Stores [output] as the output created by this transformation for asset
-  /// [id]. A transformation can output as many assets as it wants.
-  void addOutput(AssetId id, Asset output) {
-    _outputs[id] = output;
+  /// Stores [output] as the output created by this transformation.
+  ///
+  /// A transformation can output as many assets as it wants.
+  void addOutput(Asset output) {
+    // TODO(rnystrom): This should immediately throw if an output with that ID
+    // has already been created by this transformer.
+    _outputs.add(output);
   }
 }