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

src/js/icu-case-mapping.js

+// Copyright 2016 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) {
+"use strict";
+
+%CheckIsBootstrapping();
+
+var GlobalString = global.String;
+var OverrideFunction = utils.OverrideFunction;
+var ToLowerCaseI18N = utils.ImportNow("ToLowerCaseI18N");
+var ToUpperCaseI18N = utils.ImportNow("ToUpperCaseI18N");
+var ToLocaleLowerCaseI18N = utils.ImportNow("ToLocaleLowerCaseI18N");
+var ToLocaleUpperCaseI18N = utils.ImportNow("ToLocaleUpperCaseI18N");
+
+OverrideFunction(GlobalString.prototype, 'toLowerCase', ToLowerCaseI18N, true);
+OverrideFunction(GlobalString.prototype, 'toUpperCase', ToUpperCaseI18N, true);
+OverrideFunction(GlobalString.prototype, 'toLocaleLowerCase',
+                 ToLocaleLowerCaseI18N, true);
+OverrideFunction(GlobalString.prototype, 'toLocaleUpperCase',
+                 ToLocaleUpperCaseI18N, true);
+
+})