diff --git a/sdk/lib/_internal/js_runtime/lib/js_helper.dart b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
index 011e45e913c..4b21b3d90ce 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -1036,7 +1036,8 @@ class Primitives {
   }
 
   static String flattenString(String str) {
-    return JS('String', "#.charCodeAt(0) == 0 ? # : #", str, str, str);
+    return JS('returns:String;depends:none;effects:none;throws:never;gvn:true',
+              "#.charCodeAt(0) == 0 ? # : #", str, str, str);
   }
 
   static String getTimeZoneName(DateTime receiver) {