Fix filtering out .dart files in release mode.

sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart

Index: sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart
index ed36c5552025f055911885ac8d631e7224ba603c..5418b9173787d5c9dc0cf0375bafffe3b47bb516 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart
@@ -115,7 +115,11 @@ Future loadAllTransformers(BarbackServer server, PackageGraph graph,
         return unionAll(phase.map((id) => loader.transformersFor(id)));
       }).toList();
 
-      if (builtInTransformers != null && builtInTransformers.isNotEmpty) {
+      // The built-in transformers are for dart2js and forwarding assets around
+      // dart2js, and those only apply to the entrypoints in the root package.
+      if (package.name == graph.entrypoint.root.name &&
+          builtInTransformers != null &&
+          builtInTransformers.isNotEmpty) {
         phases.add(builtInTransformers);
       }