Make barback's onAsset streams broadcast rather than single-subscriber.

pkg/barback/lib/src/asset_cascade.dart

Index: pkg/barback/lib/src/asset_cascade.dart
diff --git a/pkg/barback/lib/src/asset_cascade.dart b/pkg/barback/lib/src/asset_cascade.dart
index b87ca39ae874cb26c74e72928d13b87ffcd8ba28..f0fdcb3deb1ed4ad05e6dd0d87fc815428969f76 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -182,8 +182,10 @@ class AssetCascade {
 
     if (transformers.length == 0) {
       _phases.last.updateTransformers([]);
-    } else if (transformers.length < _phases.length) {
-      _phases[transformers.length - 1].removeFollowing();
+    } else {
+      for (var i = transformers.length; i < _phases.length; i++) {
+        _phases[i].remove();
+      }
       _phases.removeRange(transformers.length, _phases.length);
     }
   }