Implement Object.getOwnPropertyDescriptor for proxies.

src/proxy.js

 // Copyright 2011 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 117 matching lines @@
     value: val,
     writable: true,
     enumerable: true,
     configurable: true});
   return true;
 }
 
 function DerivedHasTrap(name) {
   return !!this.getPropertyDescriptor(name)
 }
+
+function DerivedKeysTrap() {
+  return this.getOwnPropertyNames().filter(
+    function(name) {
+      return this.getOwnPropertyDescriptor('' + name).enumerable
+    }.bind(this));
+}