Optimize concat for one argument (two if you count 'this').

src/string.js

 // Copyright 2006-2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 69 matching lines @@
     return fast_answer;
   }
   var subject = ToString(this);
   var index = TO_INTEGER(pos);
   return %StringCharCodeAt(subject, index);
 }
 
 
 // ECMA-262, section 15.5.4.6
 function StringConcat() {
-  var len = %_ArgumentsLength() + 1;
-  var parts = new $Array(len);
-  parts[0] = IS_STRING(this) ? this : ToString(this);
-  for (var i = 1; i < len; i++) {
-    var part = %_Arguments(i - 1);
-    parts[i] = IS_STRING(part) ? part : ToString(part);
+  var len = %_ArgumentsLength();
+  var this_as_string = IS_STRING(this) ? this : ToString(this);
+  if (len === 1) {
+    return this_as_string + %_Arguments(0);
   }
-  return %StringBuilderConcat(parts, len, "");
+  var parts = new $Array(len + 1);
+  parts[0] = this_as_string;
+  for (var i = 0; i < len; i++) {
+    var part = %_Arguments(i);
+    parts[i + 1] = IS_STRING(part) ? part : ToString(part);
+  }
+  return %StringBuilderConcat(parts, len + 1, "");
 }
 
 // Match ES3 and Safari
 %FunctionSetLength(StringConcat, 1);
 
 
 // ECMA-262 section 15.5.4.7
 function StringIndexOf(searchString /* position */) {  // length == 1
   var subject_str = ToString(this);
   var pattern_str = ToString(searchString);
@@ skipping 784 matching lines @@
     "small", StringSmall,
     "strike", StringStrike,
     "sub", StringSub,
     "sup", StringSup,
     "toJSON", StringToJSON
   ));
 }
 
 
 SetupString();