Check that JSON.stringify works with BigInt objects from other realms

This is based on André Bargull's test case in
<https://bugzilla.mozilla.org/show_bug.cgi?id=1464757>.

test/built-ins/JSON/stringify/bigint-cross-realm.js

@@ -0,0 +1,19 @@
+// Copyright 2018 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-serializejsonproperty
+description: JSON.stringify called with a BigInt object from another realm
+features: [BigInt, cross-realm]
+---*/
+
+var other = $262.createRealm().global;
+var wrapped = other.Object(other.BigInt(100));
+
+assert.throws(TypeError, () => JSON.stringify(wrapped),
+              "cross-realm BigInt object without toJSON method");
+
+other.BigInt.prototype.toJSON = function () { return this.toString(); };
+
+assert.sameValue(JSON.stringify(wrapped), "\"100\"",
+                 "cross-realm BigInt object with toJSON method");