Add support for lazy transformers.

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 3454899df307c0e467193d74aa165abd342400b6..f43393f2d7546fe62d421b87b5d0ceb5314fb113 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -151,9 +151,9 @@ class AssetCascade {
       // If the requested asset is available, we can just return it.
       if (node != null && node.state.isAvailable) return node;
 
-      // If there's a build running, that build might generate the asset, so we
-      // wait for it to complete and then try again.
       if (_processDone != null) {
+        // If there's a build running, that build might generate the asset, so
+        // we wait for it to complete and then try again.
         return _processDone.then((_) => getAssetNode(id));
       }
 
@@ -233,6 +233,13 @@ class AssetCascade {
     }
   }
 
+  /// Materialize all [LazyTransformer]s' transforms in this cascade.
+  void materializeAllTransforms() {
+    for (var phase in _phases) {
+      phase.materializeAllTransforms();
+    }
+  }
+
   void reportError(BarbackException error) {
     _accumulatedErrors.add(error);
     _errorsController.add(error);