Add events in barback to bubble up the dirty bit.

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 863bac53e4991b346b06a3d982c5ed72b99ecf61..1decef7ec29157d487476b5cca314a4a017e284a 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -45,6 +45,15 @@ class TransformNode {
   /// The controllers for the asset nodes emitted by this node.
   var _outputControllers = new Map<AssetId, AssetNodeController>();
 
+  /// A stream that emits an event whenever this transform becomes dirty and
+  /// needs to be re-run.
+  ///
+  /// This may emit events when the transform was already dirty or while
+  /// processing transforms. Events are emitted synchronously to ensure that the
+  /// dirty state is thoroughly propagated as soon as any assets are changed.
+  Stream get onDirty => _onDirtyController.stream;
+  final _onDirtyController = new StreamController.broadcast(sync: true);
+
   TransformNode(this.phase, this.transformer, this.primary) {
     _primarySubscription = primary.onStateChange.listen((state) {
       if (state.isRemoved) {
@@ -80,6 +89,7 @@ class TransformNode {
     for (var controller in _outputControllers.values) {
       controller.setDirty();
     }
+    _onDirtyController.add(null);
   }
 
   /// Applies this transform.