Change String.prototype.concat to be more like similar functions.

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.
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $String = global.String;
 
 // -------------------------------------------------------------------
 
@@ skipping 43 matching lines @@
 
   var result = %_StringCharCodeAt(this, pos);
   if (!%_IsSmi(result)) {
     result = %_StringCharCodeAt(TO_STRING_INLINE(this), TO_INTEGER(pos));
   }
   return result;
 }
 
 
 // ECMA-262, section 15.5.4.6
-function StringConcat() {
+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 + %_Arguments(0);
+    return this_as_string + 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, "");
 }
 
-// Match ES3 and Safari
-%FunctionSetLength(StringConcat, 1);
-
 
 // ECMA-262 section 15.5.4.7
 function StringIndexOfJS(pattern /* position */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.indexOf");
 
   var subject = TO_STRING_INLINE(this);
   pattern = TO_STRING_INLINE(pattern);
   var index = 0;
   if (%_ArgumentsLength() > 1) {
     index = %_Arguments(1);  // position
@@ skipping 863 matching lines @@
     "fixed", StringFixed,
     "italics", StringItalics,
     "small", StringSmall,
     "strike", StringStrike,
     "sub", StringSub,
     "sup", StringSup
   ));
 }
 
 SetUpString();