Make barback more package-aware.

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';
@@ skipping 68 matching lines @@
       // 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.map((asset) => asset.id).toSet();
+      var invalidIds = outputIds
+          .where((id) => id.package != phase.graph.package).toSet();
+      outputIds.removeAll(invalidIds);
+
+      for (var id in invalidIds) {
+        // TODO(nweiz): report this as a warning rather than a failing error.
+        phase.graph.reportError(
+            new InvalidOutputException(phase.graph.package, id));
+      }
+
       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 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);
 }