Fix js test for firefox now that null becomes 'null' and not 'undefined'

sdk/lib/js/dart2js/js_dart2js.dart

Index: sdk/lib/js/dart2js/js_dart2js.dart
diff --git a/sdk/lib/js/dart2js/js_dart2js.dart b/sdk/lib/js/dart2js/js_dart2js.dart
index 8e1b371f69cde75c37ddbdf5af83a8bb82c68d08..9bf0a20f04354b046b3ddb49833412b3734116e6 100644
--- a/sdk/lib/js/dart2js/js_dart2js.dart
+++ b/sdk/lib/js/dart2js/js_dart2js.dart
@@ -455,7 +455,8 @@ class JsArray<E> extends JsObject with ListMixin<E> {
   }
 
   void sort([int compare(E a, E b)]) {
-    callMethod('sort', [compare]);
+    // Note: arr.sort(null) is a type error in FF
+    callMethod('sort', compare == null ? [] : [compare]);
   }
 }