Fix a deadlock bug with lazy transformer pipelining.

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 5eb0e6b5c36c687fd90be766fcc8ee535756f95f..96f1b606738d8b3279e37b3997ed50c70dffb8b2 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -144,6 +144,9 @@ class TransformNode {
         remove();
       } else {
         if (state.isDirty && !deferred) primary.force();
+        // If this is deferred but applying, that means it must have been
+        // forced, so we should ensure its input remains forced as well.
+        if (deferred && _state == _State.APPLYING) primary.force();
         _dirty();
       }
     });