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 af0b90c2e488565fceca6166820a36ca905b9810..16b4090334effbf86296b048b664f5b82f668260 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 +233,14 @@ class AssetCascade {
     }
   }
 
+  /// Force all [LazyTransformer]s' transforms in this cascade to begin
+  /// producing concrete assets.
+  void forceAllTransforms() {
+    for (var phase in _phases) {
+      phase.forceAllTransforms();
+    }
+  }
+
   void reportError(BarbackException error) {
     _accumulatedErrors.add(error);
     _errorsController.add(error);