Add a fast case to Array.join when all the elements and the separator are fla...

src/array.js

Index: src/array.js
===================================================================
--- src/array.js	(revision 5857)
+++ src/array.js	(working copy)
@@ -364,6 +364,10 @@
   } else if (!IS_STRING(separator)) {
     separator = ToString(separator);
   }
+
+  var result = %_FastAsciiArrayJoin(this, separator);
+  if (typeof result != "undefined") return result;
+
   var length = TO_UINT32(this.length);
   return Join(this, length, separator, ConvertToString);
 }