Add a fast case for one-element arrays in ArrayJoin

This case handles all one-element arrays that were not handled by _FastOneByteArrayJoin

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/607503004

Patch from Fadi Meawad <fmeawad@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24334 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/array.js

@@ -378,6 +378,14 @@ function ArrayJoin(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);
 }