Properly handle missing from normalized stores with keys convertible to array indices

test/mjsunit/regress/regress-509961.js

Index: test/mjsunit/regress/regress-509961.js
diff --git a/test/message/arrow-param-after-rest.js b/test/mjsunit/regress/regress-509961.js
similarity index 57%
copy from test/message/arrow-param-after-rest.js
copy to test/mjsunit/regress/regress-509961.js
index 0fade6c31d3cb6c2e82fb7027628fdfae96e1962..d28bc8a268dcba150b511f02d17672989176e420 100644
--- a/test/message/arrow-param-after-rest.js
+++ b/test/mjsunit/regress/regress-509961.js
@@ -1,7 +1,10 @@
 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
-//
-// Flags: --harmony-rest-parameters --harmony-arrow-functions
 
-(...x, y) => 10
+var o = { x: 0 };
+delete o.x;
+function store(o, p, v) { o[p] = v; }
+store(o, "x", 1);
+store(o, "x", 1);
+store(o, "0", 1);