Support actually removing phases in barback.

pkg/barback/lib/src/asset_cascade.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library barback.asset_cascade;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 168 matching lines @@
 
     for (var i = 0; i < transformers.length; i++) {
       if (_phases.length > i) {
         _phases[i].updateTransformers(transformers[i]);
         continue;
       }
 
       _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);
     }
   }
 
   void reportError(BarbackException error) {
     _accumulatedErrors.add(error);
     _errorsController.add(error);
   }
 
   /// Add [phase] to the end of [_phases] and watch its [onDirty] stream.
   void _addPhase(Phase phase) {
@@ skipping 55 matching lines @@
 
         // Otherwise, everything is done.
         return;
       }
 
       // Process that phase and then loop onto the next.
       return future.then((_) => _process());
     });
   }
 }