don't run dart gen on JS patch code

test/dart_codegen/expect/_internal/list.dart

 part of dart._internal;
 
 abstract class FixedLengthListMixin<E> {
   void set length(int newLength) {
     throw new UnsupportedError(
         "Cannot change the length of a fixed-length list");
   }
   void add(E value) {
     throw new UnsupportedError("Cannot add to a fixed-length list");
   }
@@ skipping 160 matching lines @@
       new UnsupportedError("Cannot remove from unmodifiable List");
 }
 abstract class NonGrowableListError {
   static UnsupportedError add() =>
       new UnsupportedError("Cannot add to non-growable List");
   static UnsupportedError length() =>
       new UnsupportedError("Cannot change length of non-growable List");
   static UnsupportedError remove() =>
       new UnsupportedError("Cannot remove from non-growable List");
 }
-List makeListFixedLength(List growableList) {
-  JSArray.markFixedList(growableList);
-  return growableList;
-}
+external List makeListFixedLength(List growableList);