Make barback more package-aware.

pkg/barback/lib/src/transform_node.dart

Index: pkg/barback/lib/src/transform_node.dart
diff --git a/pkg/barback/lib/src/transform_node.dart b/pkg/barback/lib/src/transform_node.dart
index 194f07333959701b932f3cf3df50315e74078015..c1bbcfd7df0f88c17fb7a610d0af3bf65ede7932 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -86,6 +86,17 @@ class TransformNode {
 
       // 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 = outputIds.removeAll(invalidIds);
+
+      // TODO(nweiz): if there are invalid ids, do we consider the transform to
+      // have failed?

[Bob Nystrom, 2013/07/16 17:36:01]

Probably not, I think.

[nweiz, 2013/07/16 19:39:39]

Right now we have no means of reporting a bad condition without considering it
an error. I'll update the TODO for that.

+      for (var id in invalidIds) {
+        phase.graph.reportError(
+            new InvalidOutputException(phase.graph.package, id));
+      }
+
       var removed = _outputs.difference(outputIds);
       _outputs = outputIds;