[es5] call ToString() on argument in String.prototype.concat() fast case

src/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.
 
 var $stringCharAt;
 var $stringIndexOf;
 var $stringSubstring;
 
 (function() {
 
@@ skipping 54 matching lines @@
   if (!%_IsSmi(result)) {
     result = %_StringCharCodeAt(TO_STRING_INLINE(this), TO_INTEGER(pos));
   }
   return result;
 }
 
 
 // ECMA-262, section 15.5.4.6
 function StringConcat(other /* and more */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.concat");
-
   var len = %_ArgumentsLength();
   var this_as_string = TO_STRING_INLINE(this);
   if (len === 1) {
-    return this_as_string + other;
+    return this_as_string + TO_STRING_INLINE(other);
   }
   var parts = new InternalArray(len + 1);
   parts[0] = this_as_string;
   for (var i = 0; i < len; i++) {
     var part = %_Arguments(i);
     parts[i + 1] = TO_STRING_INLINE(part);
   }
   return %StringBuilderConcat(parts, len + 1, "");
 }
 
@@ skipping 1086 matching lines @@
   "strike", StringStrike,
   "sub", StringSub,
   "sup", StringSup
 ));
 
 $stringCharAt = StringCharAtJS;
 $stringIndexOf = StringIndexOfJS;
 $stringSubstring = StringSubstring;
 
 })();