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

R=sra@google.com

Review URL: https://codereview.chromium.org//438503003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@38821 260f80e4-7a28-3924-810f-c04153c831b5

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]);
   }
 }