Add support for transformer clusters to 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 eb65679b27f7de70e02592f418ea9022f6c9042f..6bc27bcbe87246eae44c4f125ff26a46e5e05da7 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -99,7 +99,8 @@ class AssetCascade {
   var _newChanges = false;
 
   /// Returns all currently-available output assets from this cascade.
-  AssetSet get availableOutputs => _phases.last.availableOutputs;
+  AssetSet get availableOutputs =>
+    new AssetSet.from(_phases.last.availableOutputs.map((node) => node.asset));
 
   /// Creates a new [AssetCascade].
   ///
@@ -189,24 +190,24 @@ class AssetCascade {
     });
   }
 
-  /// Sets this cascade's transformer phases to [transformers].
-  void updateTransformers(Iterable<Iterable<Transformer>> transformers) {
-    transformers = transformers.toList();
+  /// Sets this cascade's operator phases to [operators].
+  void updateOperators(Iterable<Iterable<Operator>> operators) {
+    operators = operators.toList();
 
-    for (var i = 0; i < transformers.length; i++) {
+    for (var i = 0; i < operators.length; i++) {
       if (_phases.length > i) {
-        _phases[i].updateTransformers(transformers[i]);
+        _phases[i].updateOperators(operators[i]);
         continue;
       }
 
-      _addPhase(_phases.last.addPhase(transformers[i]));
+      _addPhase(_phases.last.addPhase(operators[i]));
     }
 
-    if (transformers.length == 0) {
-      _phases.last.updateTransformers([]);
-    } else if (transformers.length < _phases.length) {
-      _phases[transformers.length - 1].removeFollowing();
-      _phases.removeRange(transformers.length, _phases.length);
+    if (operators.length == 0) {
+      _phases.last.updateOperators([]);
+    } else if (operators.length < _phases.length) {
+      _phases[operators.length - 1].removeFollowing();
+      _phases.removeRange(operators.length, _phases.length);
     }
   }