[stubs] Fix negative index lookup in hasOwnProperty

test/mjsunit/regress/regress-crbug-673008.js

+// Copyright 2016 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.
+
+var a = {
+  "33": true,
+  "-1": true
+};
+
+var strkeys = Object.keys(a).map(function(k) { return "" + k });
+var numkeys = Object.keys(a).map(function(k) { return +k });
+var keys = strkeys.concat(numkeys);
+
+keys.forEach(function(k) {
+  assertTrue(a.hasOwnProperty(k),
+             "property not found: " + k + "(" + (typeof k) + ")");
+});
+
+var b = {};
+b.__proto__ = a;
+keys.forEach(function(k) {
+  assertTrue(k in b, "property not found: " + k + "(" + (typeof k) + ")");
+});