Implement Object.getOwnPropertyDescriptor for proxies.

src/proxy.js

Index: src/proxy.js
diff --git a/src/proxy.js b/src/proxy.js
index cb9c020e35c60c9b0cdcf15e961ca9a8fdb73789..72b5c225652a71494c24d671bfcc67b5ad86891f 100644
--- a/src/proxy.js
+++ b/src/proxy.js
@@ -135,3 +135,10 @@ function DerivedSetTrap(receiver, name, val) {
 function DerivedHasTrap(name) {
   return !!this.getPropertyDescriptor(name)
 }
+
+function DerivedKeysTrap() {
+  return this.getOwnPropertyNames().filter(
+    function(name) {
+      return this.getOwnPropertyDescriptor('' + name).enumerable
+    }.bind(this));
+}