[json] implement JSON.stringify gap pre-processing in C++.

src/js/json.js

Index: src/js/json.js
diff --git a/src/js/json.js b/src/js/json.js
index 456b20e7c38dec3267986b9ecb60d77f3b73b4d3..e2f7646f21e7309b3e0daf918b45f48dd9cb08e9 100644
--- a/src/js/json.js
+++ b/src/js/json.js
@@ -233,6 +233,9 @@ function JSONStringify(value, replacer, space) {
       space = TO_STRING(space);
     }
   }
+  if (!IS_CALLABLE(replacer) && !property_list) {
+    return %BasicJSONStringify(value, space);
+  }
   var gap;
   if (IS_NUMBER(space)) {
     space = MaxSimple(0, MinSimple(TO_INTEGER(space), 10));
@@ -246,9 +249,6 @@ function JSONStringify(value, replacer, space) {
   } else {
     gap = "";
   }
-  if (!IS_CALLABLE(replacer) && !property_list) {
-    return %BasicJSONStringify(value, gap);
-  }
   return JSONSerialize('', {'': value}, replacer, new Stack(), "", gap);
 }