Make Assets know their ID.

pkg/barback/lib/src/transform_node.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_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_graph.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
+import 'asset_set.dart';
 import 'errors.dart';
 import 'phase.dart';
 import 'transform.dart';
 import 'transformer.dart';
 
 /// Describes a transform on a set of assets and its relationship to the build
 /// dependency graph.
 ///
 /// Keeps track of whether it's dirty and needs to be run and which assets it
 /// depends on.
@@ skipping 28 matching lines @@
   void dirty() {
     _isDirty = true;
   }
 
   /// Applies this transform.
   ///
   /// Returns a [TransformOutputs] describing the resulting outputs compared to
   /// previous runs.
   Future<TransformOutputs> apply() {
     var newInputs = new Set<AssetNode>();
-    var newOutputs = new Map<AssetId, Asset>();
+    var newOutputs = new AssetSet();
     var transform = createTransform(this, newInputs, newOutputs);
     return _transformer.apply(transform).catchError((error) {
       // Catch all transformer errors and pipe them to the results stream.
       // This is so a broken transformer doesn't take down the whole graph.
       phase.graph.reportError(error);
 
       // Don't allow partial results from a failed transform.
       newOutputs.clear();
     }).then((_) {
       _isDirty = false;
 
       // Stop watching any inputs that were removed.
       for (var oldInput in _inputs) {
         oldInput.consumers.remove(this);
       }
 
       // Watch any new inputs so this transform will be re-processed when an
       // input is modified.
       for (var newInput in newInputs) {
         newInput.consumers.add(this);
       }
 
       _inputs = newInputs;
 
       // See which outputs are missing from the last run.
-      var outputIds = newOutputs.keys.toSet();
+      var outputIds = newOutputs.map((asset) => asset.id).toSet();
       var removed = _outputs.difference(outputIds);
       _outputs = outputIds;
 
       return new TransformOutputs(newOutputs, removed);
     });
   }
 }
 
 /// The result of running a [Transform], compared to the previous time it was
 /// applied.
 class TransformOutputs {
   /// The outputs that are new or were modified since the last run.
-  final Map<AssetId, Asset> updated;
+  final AssetSet updated;
 
   /// The outputs that were created by the previous run but were not generated
   /// by the most recent run.
   final Set<AssetId> removed;
 
   TransformOutputs(this.updated, this.removed);
 }