Introduce new runtime function to make join with lower memory usage.

src/array.js

 // Copyright 2010 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 143 matching lines @@
         if (IS_NUMBER(e)) elements[i] = %_NumberToString(e);
         else {
           if (!IS_STRING(e)) e = convert(e);
           elements[i] = e;
         }
       }
     }   
     var result = %_FastAsciiArrayJoin(elements, separator);
     if (!IS_UNDEFINED(result)) return result;   
 
-    var length2 = (length << 1) - 1;
-    var j = length2;
-    var i = length;
-    elements[--j] = elements[--i];
-    while (i > 0) {
-      elements[--j] = separator;
-      elements[--j] = elements[--i];
-    }
-    return %StringBuilderConcat(elements, length2, '');    
+    return %StringBuilderJoin(elements, length, separator);
   } finally {
     // Make sure to remove the last element of the visited array no
     // matter what happens.
     if (is_array) visited_arrays.length = visited_arrays.length - 1;
   }
 }
 
 
 function ConvertToString(x) {
   // Assumes x is a non-string. 
@@ skipping 1048 matching lines @@
     "lastIndexOf", getFunction("lastIndexOf", ArrayLastIndexOf, 1),
     "reduce", getFunction("reduce", ArrayReduce, 1),
     "reduceRight", getFunction("reduceRight", ArrayReduceRight, 1)
   ));
 
   %FinishArrayPrototypeSetup($Array.prototype);
 }
 
 
 SetupArray();