Minor change to for-in...

src/runtime.js

Index: src/runtime.js
===================================================================
--- src/runtime.js	(revision 5238)
+++ src/runtime.js	(working copy)
@@ -387,11 +387,11 @@
 
 // Filter a given key against an object by checking if the object
 // has a property with the given key; return the key as a string if
-// it has. Otherwise returns null. Used in for-in statements.
+// it has. Otherwise returns 0 (smi). Used in for-in statements.
 function FILTER_KEY(key) {
   var string = %ToString(key);
   if (%HasProperty(this, string)) return string;
-  return null;
+  return 0;
 }