Use ICU case conversion/transliterator for case conversion behind a flag

src/js/prologue.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function(global, utils, extrasUtils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
 
@@ skipping 168 matching lines @@
     imports(exports_container);
   }
 
   // Whitelist of exports from normal natives to experimental natives and debug.
   var expose_list = [
     "ArrayToString",
     "ErrorToString",
     "GetIterator",
     "GetMethod",
     "IsNaN",
+    "LocaleConvertCase",
     "MakeError",
     "MakeRangeError",
     "MakeTypeError",
     "MapEntries",
     "MapIterator",
     "MapIteratorNext",
     "MaxSimple",
     "MinSimple",
     "NumberIsInteger",
     "PromiseChain",
@@ skipping 153 matching lines @@
 
 extrasUtils.uncurryThis = function uncurryThis(func) {
   return function(thisArg, ...args) {
     return %reflect_apply(func, thisArg, args);
   };
 };
 
 %ToFastProperties(extrasUtils);
 
 })