[stubs] Port String.prototype.substring to TurboFan

src/js/string.js

Index: src/js/string.js
diff --git a/src/js/string.js b/src/js/string.js
index a6dc2f082d3d6c04c8c718eac688790f2de90791..9262a161dae1b58edf73348117f3cd92b941dd91 100644
--- a/src/js/string.js
+++ b/src/js/string.js
@@ -17,6 +17,7 @@ var IsRegExp;
 var MaxSimple;
 var MinSimple;
 var RegExpInitialize;
+var StringSubstring = GlobalString.prototype.substring;
 var matchSymbol = utils.ImportNow("match_symbol");
 var replaceSymbol = utils.ImportNow("replace_symbol");
 var searchSymbol = utils.ImportNow("search_symbol");
@@ -237,40 +238,6 @@ function StringSplitJS(separator, limit) {
   return %StringSplit(subject, separator_string, limit);
 }
 
-
-// ECMA-262 section 15.5.4.15
-function StringSubstring(start, end) {
-  CHECK_OBJECT_COERCIBLE(this, "String.prototype.subString");
-
-  var s = TO_STRING(this);
-  var s_len = s.length;
-
-  var start_i = TO_INTEGER(start);
-  if (start_i < 0) {
-    start_i = 0;
-  } else if (start_i > s_len) {
-    start_i = s_len;
-  }
-
-  var end_i = s_len;
-  if (!IS_UNDEFINED(end)) {
-    end_i = TO_INTEGER(end);
-    if (end_i > s_len) {
-      end_i = s_len;
-    } else {
-      if (end_i < 0) end_i = 0;
-      if (start_i > end_i) {
-        var tmp = end_i;
-        end_i = start_i;
-        start_i = tmp;
-      }
-    }
-  }
-
-  return %_SubString(s, start_i, end_i);
-}
-
-
 // ecma262/#sec-string.prototype.substr
 function StringSubstr(start, length) {
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.substr");
@@ -591,7 +558,6 @@ utils.InstallFunctions(GlobalString.prototype, DONT_ENUM, [
   "search", StringSearch,
   "slice", StringSlice,
   "split", StringSplitJS,
-  "substring", StringSubstring,
   "substr", StringSubstr,
   "startsWith", StringStartsWith,
   "toLowerCase", StringToLowerCaseJS,