Add support for transformer clusters to barback.

pkg/barback/lib/src/phase_input.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_input;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 90 matching lines @@
     _onDirtyController.add(null);
     _onDirtyPool.close();
     _inputForwarder.close();
     if (_passThroughController != null) {
       _passThroughController.setRemoved();
       _passThroughController = null;
     }
   }
 
   /// Set this input's transformers to [transformers].
-  void updateTransformers(Set<Transformer> newTransformers) {
+  void updateTransformers(Iterable<Transformer> newTransformers) {
+    newTransformers = newTransformers.toSet();
     var oldTransformers = _transformers.toSet();
     for (var removedTransformer in
          oldTransformers.difference(newTransformers)) {
       _transformers.remove(removedTransformer);
 
       // If the transformers are being adjusted for [id], it will
       // automatically pick up on [removedTransformer] being gone.
       if (_adjustTransformersFuture != null) continue;
 
       _transforms.removeWhere((transform) {
         if (transform.transformer != removedTransformer) return false;
         transform.remove();
         return true;
       });
     }
 
     if (_transforms.isEmpty && _adjustTransformersFuture == null &&
         _passThroughController == null) {
-      _passThroughController =
-          new AssetNodeController.available(input.asset, input.transform);
+      _passThroughController = new AssetNodeController.from(input);
       _newPassThrough = true;
     }
 
     var brandNewTransformers = newTransformers.difference(oldTransformers);
     if (brandNewTransformers.isEmpty) return;
 
     brandNewTransformers.forEach(_transformers.add);
     if (_adjustTransformersFuture == null) _adjustTransformers();
   }
 
@@ skipping 91 matching lines @@
   ///
   /// If [input] was already passed-through, this will update the passed-through
   /// value.
   void _adjustPassThrough() {
     assert(input.state.isAvailable);
 
     if (_transforms.isEmpty) {
       if (_passThroughController != null) {
         _passThroughController.setAvailable(input.asset);
       } else {
-        _passThroughController =
-            new AssetNodeController.available(input.asset, input.transform);
+        _passThroughController = new AssetNodeController.from(input);
         _newPassThrough = true;
       }
     } else if (_passThroughController != null) {
       _passThroughController.setRemoved();
       _passThroughController = null;
       _newPassThrough = false;
     }
   }
 
   /// Like [AssetNode.tryUntilStable], but also re-runs [callback] if this
@@ skipping 33 matching lines @@
       return new Future.value(
           new Set<AssetNode>.from([_passThroughController.node]));
     }
 
     return Future.wait(_transforms.map((transform) {
       if (!transform.isDirty) return new Future.value(new Set());
       return transform.apply();
     })).then((outputs) => unionAll(outputs));
   }
 }