Add a fast case for one-element arrays in ArrayJoin

src/array.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.
 
 "use strict";
 
 // This file relies on the fact that the following declarations have been made
 // in runtime.js:
 // var $Array = global.Array;
 
@@ skipping 360 matching lines @@
   var length = TO_UINT32(array.length);
   if (IS_UNDEFINED(separator)) {
     separator = ',';
   } else if (!IS_STRING(separator)) {
     separator = NonStringToString(separator);
   }
 
   var result = %_FastOneByteArrayJoin(array, separator);
   if (!IS_UNDEFINED(result)) return result;
 
+  // Fast case for one-element arrays.
+  if (length === 1) {
+    var e = array[0];
+    if (IS_STRING(e)) return e;
+    if (IS_NULL_OR_UNDEFINED(e)) return '';
+    return NonStringToString(e);
+  }
+
   return Join(array, length, separator, ConvertToString);
 }
 
 
 function ObservedArrayPop(n) {
   n--;
   var value = this[n];
 
   try {
     BeginPerformSplice(this);
@@ skipping 1156 matching lines @@
   ));
 
   SetUpLockedPrototype(InternalPackedArray, $Array(), $Array(
     "join", getFunction("join", ArrayJoin),
     "pop", getFunction("pop", ArrayPop),
     "push", getFunction("push", ArrayPush)
   ));
 }
 
 SetUpArray();