[json] handle proxies in BasicJsonSerializer.

src/js/json.js

Index: src/js/json.js
diff --git a/src/js/json.js b/src/js/json.js
index 09fe688ebb1dca2755bb11e83329a5b68c749517..f5c0ae7a3cb7219d7d106bbf27205939871fce0c 100644
--- a/src/js/json.js
+++ b/src/js/json.js
@@ -201,9 +201,7 @@ function JSONSerialize(key, holder, replacer, stack, indent, gap) {
 
 
 function JSONStringify(value, replacer, space) {
-  if (arguments.length === 1 && !IS_PROXY(value)) {
-    return %BasicJSONStringify(value, "");
-  }
+  if (arguments.length === 1) return %BasicJSONStringify(value, "");
   if (!IS_CALLABLE(replacer) && %is_arraylike(replacer)) {
     var property_list = new InternalArray();
     var seen_properties = new GlobalSet();
@@ -248,7 +246,7 @@ function JSONStringify(value, replacer, space) {
   } else {
     gap = "";
   }
-  if (!IS_CALLABLE(replacer) && !property_list && !IS_PROXY(value)) {
+  if (!IS_CALLABLE(replacer) && !property_list) {
     return %BasicJSONStringify(value, gap);
   }
   return JSONSerialize('', {'': value}, replacer, new Stack(), "", gap);