Support actually removing phases in barback.

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 e8e9355fe17d00198b628b29d6aa3014d1e79618..078c510c530322a16c6e59808f4a4922525c6911 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -186,12 +186,11 @@ class AssetCascade {
       _addPhase(_phases.last.addPhase(transformers[i]));
     }
 
-    if (transformers.length < _phases.length) {
-      for (var i = transformers.length; i < _phases.length; i++) {
-        // TODO(nweiz): actually remove phases rather than emptying them of
-        // transformers.
-        _phases[i].updateTransformers([]);
-      }
+    if (transformers.length == 0) {
+      _phases.last.updateTransformers([]);
+    } else if (transformers.length < _phases.length) {
+      _phases[transformers.length - 1].removeFollowing();
+      _phases.removeRange(transformers.length, _phases.length);
     }
   }