Implement Object.keys for proxies.

src/proxy.js

Index: src/proxy.js
diff --git a/src/proxy.js b/src/proxy.js
index cb9c020e35c60c9b0cdcf15e961ca9a8fdb73789..27524bd91849ae8b880a676133c6184b3b41470b 100644
--- a/src/proxy.js
+++ b/src/proxy.js
@@ -135,3 +135,15 @@ function DerivedSetTrap(receiver, name, val) {
 function DerivedHasTrap(name) {
   return !!this.getPropertyDescriptor(name)
 }
+
+function DerivedKeysTrap() {
+  var names = this.getOwnPropertyNames()
+  var enumerableNames = []
+  for (var i = 0, count = 0; i < names.length; ++i) {
+    var name = names[i]
+    if (this.getOwnPropertyDescriptor(TO_STRING_INLINE(name)).enumerable) {
+      enumerableNames[count++] = names[i]
+    }
+  }
+  return enumerableNames
+}