Add new test for receiver of toJSON on bigint values (#2079)

Spec link:

  https://tc39.github.io/proposal-bigint/#sec-serializejsonproperty

Thanks to André Bargull for the test:

  https://bugzilla.mozilla.org/show_bug.cgi?id=1528784

test/built-ins/JSON/stringify/bigint-tojson-receiver.js

@@ -0,0 +1,25 @@
+// Copyright 2019 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+// Test case written by André Bargull.
+
+/*---
+esid: sec-serializejsonproperty
+description: toJSON method called with BigInt as receiver
+features: [BigInt]
+---*/
+
+assert.throws(TypeError, () => JSON.stringify(1n),
+              "toString throws for BigInt object");
+
+// The BigInt proposal changes the SerializeJSONProperty algorithm to
+// specifically allow passing BigInt objects as receivers for the toJSON
+// method.
+Object.defineProperty(BigInt.prototype, "toJSON", {
+    get() {
+        "use strict";
+        return () => typeof this;
+    }
+});
+
+assert.sameValue(JSON.stringify(1n), "\"bigint\"",
+                 "BigInt toJSON method called with value as receiver");