[builtins] Move StringIndexOf to a C++ builtin.

src/js/i18n.js

Index: src/js/i18n.js
diff --git a/src/js/i18n.js b/src/js/i18n.js
index 5074404bb13cf65750da294d7f8767387565932c..5948ac40e9fd19d7779e05817975d7d253b16041 100644
--- a/src/js/i18n.js
+++ b/src/js/i18n.js
@@ -33,7 +33,6 @@ var OverrideFunction = utils.OverrideFunction;
 var patternSymbol = utils.ImportNow("intl_pattern_symbol");
 var resolvedSymbol = utils.ImportNow("intl_resolved_symbol");
 var SetFunctionName = utils.SetFunctionName;
-var StringIndexOf;
 var StringSubstr;
 var StringSubstring;
 
@@ -42,7 +41,6 @@ utils.Import(function(from) {
   ArrayPush = from.ArrayPush;
   InternalRegExpMatch = from.InternalRegExpMatch;
   InternalRegExpReplace = from.InternalRegExpReplace;
-  StringIndexOf = from.StringIndexOf;
   StringSubstr = from.StringSubstr;
   StringSubstring = from.StringSubstring;
 });
@@ -829,7 +827,7 @@ function isStructuallyValidLanguageTag(locale) {
   }
 
   // Just return if it's a x- form. It's all private.
-  if (%_Call(StringIndexOf, locale, 'x-') === 0) {
+  if (%StringIndexOf(locale, 'x-', 0) === 0) {
     return true;
   }
 
@@ -2060,7 +2058,7 @@ function LocaleConvertCase(s, locales, isToUpper) {
   }
 
   // StringSplit is slower than this.
-  var pos = %_Call(StringIndexOf, language, '-');
+  var pos = %StringIndexOf(language, '-', 0);
   if (pos != -1) {
     language = %_Call(StringSubstring, language, 0, pos);
   }