Implement for-in loop for proxies.

src/v8natives.js

Index: src/v8natives.js
diff --git a/src/v8natives.js b/src/v8natives.js
index 56da2a47ed08e1973f1d4487c5175af1682adfc2..e6669d58aef496483e705b8467de777d0a835b0a 100644
--- a/src/v8natives.js
+++ b/src/v8natives.js
@@ -937,14 +937,14 @@ function ToStringArray(obj, trap) {
   }
   var n = ToUint32(obj.length);
   var array = new $Array(n);
-  var names = {}
+  var names = {}  // TODO(rossberg): use sets once they are ready.
   for (var index = 0; index < n; index++) {
     var s = ToString(obj[index]);
     if (s in names) {
       throw MakeTypeError("proxy_repeated_prop_name", [obj, trap, s])
     }
     array[index] = s;
-    names.s = 0;
+    names[s] = 0;
   }
   return array;
 }