Fix a barback bug that triggered when an asset was removed.

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 b17c9a5ba84d98898a9a372b268662061d480734..84e42c0d0a5664bf10f2189bff6578099b1ed210 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -43,6 +43,7 @@ class TransformNode {
   /// The outputs created by this transform the last time it was run.
   ///
   /// Used to tell if an output was removed in a later run.
+  Set<AssetId> get outputs => _outputs;

[Bob Nystrom, 2013/07/17 17:18:26]

Instead of making this public, how about defining:

TransformOutputs removeAllOutputs() => new TransformOutputs(new AssetSet(),
_outputs);

[nweiz, 2013/07/17 18:29:55]

I thought about that, but I couldn't come up with a name I was happy with and I
took that as a sign that that wasn't the right way to do it. For example,
[removeAllOutputs] sounds like it's a mutating operation that will remove
outputs, which isn't the case.

I'm not too worried about making [outputs] public; I think it and [inputs] will
both need to be public eventually to support more intelligence about what
transforms need to be waited for when requesting an asset.

   var _outputs = new Set<AssetId>();
 
   TransformNode(this.phase, this._transformer, this.primary);