Inctroduce NewStrictSubstring to avoid check for SubString(str, 0, str.length...

src/json.js

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 260 matching lines @@
 
 
 function BasicJSONSerialize(key, value, stack, builder) {
   if (IS_SPEC_OBJECT(value)) {
     var toJSON = value.toJSON;
     if (IS_FUNCTION(toJSON)) {
       value = %_CallFunction(value, ToString(key), toJSON);
     }
   }
   if (IS_STRING(value)) {
-    builder.push(%QuoteJSONString(value));
+    builder.push(value !== "" ? %QuoteJSONString(value) : '""');
   } else if (IS_NUMBER(value)) {
     builder.push(JSON_NUMBER_TO_STRING(value));
   } else if (IS_BOOLEAN(value)) {
     builder.push(value ? "true" : "false");
   } else if (IS_NULL(value)) {
     builder.push("null");
   } else if (IS_SPEC_OBJECT(value) && !(typeof value == "function")) {
     // Value is a non-callable object.
     // Unwrap value if necessary
     if (IS_NUMBER_WRAPPER(value)) {
@@ skipping 46 matching lines @@
 }
 
 function SetupJSON() {
   InstallFunctions($JSON, DONT_ENUM, $Array(
     "parse", JSONParse,
     "stringify", JSONStringify
   ));
 }
 
 SetupJSON();