[intl] Check for duplicate BCP 47 tags in a case-insensitive way

src/js/i18n.js

 // Copyright 2013 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.
 
 // ECMAScript 402 API implementation.
 
 /**
  * Intl object is a single object that has some named properties,
  * all of which are constructors.
  */
@@ skipping 833 matching lines @@
  * hant-cmn-cn, through.
  *
  * Returns false if the language tag is invalid.
  */
 function isStructuallyValidLanguageTag(locale) {
   // Check if it's well-formed, including grandfadered tags.
   if (IS_NULL(%regexp_internal_match(GetLanguageTagRE(), locale))) {
     return false;
   }
 
+  locale = %StringToLowerCase(locale);
+
   // Just return if it's a x- form. It's all private.
   if (%StringIndexOf(locale, 'x-', 0) === 0) {
     return true;
   }
 
   // Check if there are any duplicate variants or singletons (extensions).
 
   // Remove private use section.
   locale = %StringSplit(locale, '-x-', kMaxUint32)[0];
 
@@ skipping 1302 matching lines @@
   }
 );
 
 %FunctionRemovePrototype(FormatDateToParts);
 
 utils.Export(function(to) {
   to.FormatDateToParts = FormatDateToParts;
 });
 
 })