Better handling of asset collisions in barback.

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 a7241aed2ef2dffcab7a16ac393793ae7f61640d..011345de5746ce2fe32d00d0c0151e09c6dcfa72 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -98,6 +98,8 @@ class TransformNode {
   /// Returns a set of asset nodes representing the outputs from this transform
   /// that weren't emitted last time it was run.
   Future<Set<AssetNode>> apply() {
+    assert(!_onDirtyController.isClosed);
+
     var newOutputs = new AssetSet();
     var transform = createTransform(this, newOutputs);