Fix an analyzer warning in pub.

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

Index: sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
index 452291bc79f2f428f4bda9dbb707e8e9e66b70dc..44908dcca11d9b871d4adf48f2abfb5c6dfaee13 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -626,7 +626,6 @@ Future _call(SendPort port, message) {
 /// An [AssetNotFoundException] that was originally raised in another isolate. 
 class _CrossIsolateAssetNotFoundException extends dart.CrossIsolateException
     implements AssetNotFoundException {
-  final TransformInfo transform;
   final AssetId id;
 
   String get message => "Could not find asset $id.";