Aviod double checking IS_STRING when joining arrays.

src/v8natives.js

Index: src/v8natives.js
===================================================================
--- src/v8natives.js	(revision 6219)
+++ src/v8natives.js	(working copy)
@@ -1156,7 +1156,7 @@
   if (n > 1) {
     p = new $Array(n - 1);
     for (var i = 0; i < n - 1; i++) p[i] = %_Arguments(i);
-    p = Join(p, n - 1, ',', ToString);
+    p = Join(p, n - 1, ',', NonStringToString);
     // If the formal parameters string include ) - an illegal
     // character - it may make the combined function expression
     // compile. We avoid this problem by checking for this early on.