[csa] Migrate String.prototype.concat to TurboFan builtin.

src/js/string.js

 // Copyright 2012 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) {
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
 
 var GlobalString = global.String;
 var matchSymbol = utils.ImportNow("match_symbol");
 var searchSymbol = utils.ImportNow("search_symbol");
 
 //-------------------------------------------------------------------
 
-// ECMA-262, section 15.5.4.6
-function StringConcat(other /* and more */) {  // length == 1
-  "use strict";
-  CHECK_OBJECT_COERCIBLE(this, "String.prototype.concat");
-  var s = TO_STRING(this);
-  var len = arguments.length;
-  for (var i = 0; i < len; ++i) {
-    s = s + TO_STRING(arguments[i]);
-  }
-  return s;
-}
-
-
 // ES6 21.1.3.11.
 function StringMatchJS(pattern) {
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.match");
 
   if (!IS_NULL_OR_UNDEFINED(pattern)) {
     var matcher = pattern[matchSymbol];
     if (!IS_UNDEFINED(matcher)) {
       return %_Call(matcher, pattern, this);
     }
   }
@@ skipping 240 matching lines @@
 // -------------------------------------------------------------------
 
 // Set up the non-enumerable functions on the String object.
 utils.InstallFunctions(GlobalString, DONT_ENUM, [
   "raw", StringRaw
 ]);
 
 // Set up the non-enumerable functions on the String prototype object.
 utils.InstallFunctions(GlobalString.prototype, DONT_ENUM, [
   "codePointAt", StringCodePointAt,
-  "concat", StringConcat,
   "match", StringMatchJS,
   "repeat", StringRepeat,
   "search", StringSearch,
   "slice", StringSlice,
 
   "link", StringLink,
   "anchor", StringAnchor,
   "fontcolor", StringFontcolor,
   "fontsize", StringFontsize,
   "big", StringBig,
   "blink", StringBlink,
   "bold", StringBold,
   "fixed", StringFixed,
   "italics", StringItalics,
   "small", StringSmall,
   "strike", StringStrike,
   "sub", StringSub,
   "sup", StringSup
 ]);
 
 })