Only run at most 10 transformers at once in barback.

pkg/barback/lib/src/transform_node.dart

Index: pkg/barback/lib/src/transform_node.dart
diff --git a/pkg/barback/lib/src/transform_node.dart b/pkg/barback/lib/src/transform_node.dart
index 6df431540abf424db33bf94c2efa608a75a01b7e..e93b30ed7eeab94f701a0046c81ec5f88c9efff3 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -125,7 +125,10 @@ class TransformNode {
     _inputSubscriptions.clear();
 
     _isDirty = false;
-    return transformer.apply(transform).catchError((error) {
+
+    return phase.cascade.graph.transformPool
+        .withResource(() => transformer.apply(transform))
+        .catchError((error) {
       // If the transform became dirty while processing, ignore any errors from
       // it.
       if (_isDirty) return;