Support actually removing phases in barback.

pkg/barback/lib/src/phase.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.phase;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset_cascade.dart';
@@ skipping 135 matching lines @@
       output.removeListeners();
 
       // Removing [output]'s listeners will cause it to be removed from
       // [_outputs], so we have to put it back.
       _outputs[output.output.id] = output;
       _next.addInput(output.output);
     }
     return _next;
   }
 
+  /// Mark this phase as removed.
+  ///
+  /// This will remove all the phase's outputs and all following phases.
+  void remove() {
+    removeFollowing();
+    for (var input in _inputs.values.toList()) {
+      input.remove();
+    }
+    _onDirtyPool.close();
+  }
+
+  /// Remove all phases after this one.
+  Phase removeFollowing() {
+    if (_next == null) return;
+    _next.remove();
+    _next = null;
+  }
+
   /// Processes this phase.
   ///
   /// Returns a future that completes when processing is done. If there is
   /// nothing to process, returns `null`.
   Future process() {
     if (!_inputs.values.any((input) => input.isDirty)) return null;
 
     var outputIds = new Set<AssetId>();
     return Future.wait(_inputs.values.map((input) {
       if (!input.isDirty) return new Future.value(new Set());
@@ skipping 17 matching lines @@
       for (var id in outputIds) {
         // It's possible the output was removed before other transforms in this
         // phase finished.
         if (!_outputs.containsKey(id)) continue;
         var exception = _outputs[id].collisionException;
         if (exception != null) cascade.reportError(exception);
       }
     });
   }
 }