Don't load built-in transformers for "pub run".

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

Index: sdk/lib/_internal/pub/lib/src/barback/dependency_computer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dependency_computer.dart b/sdk/lib/_internal/pub/lib/src/barback/dependency_computer.dart
index bac5ab5adb3ea5fbc5b42fba7ad74dc5c06514de..0105cbfae020728f3fe777fdcb6992d819d94dba 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dependency_computer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dependency_computer.dart
@@ -96,7 +96,8 @@ class DependencyComputer {
   Set<TransformerId> transformersNeededByLibrary(AssetId id) {
     var library = _graph.packages[id.package].path(p.fromUri(id.path));
     _loadPackageComputer(id.package);
-    return _packageComputers[id.package].transformersNeededByLibrary(library);
+    return _packageComputers[id.package].transformersNeededByLibrary(library)
+        .where((id) => !id.isBuiltInTransformer).toSet();
   }
 
   /// Returns the set of all transformers that need to be loaded before [id] is