diff -r 000000000000 -r 6474c204b198 js/xpconnect/tests/unit/test_bug780370.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/xpconnect/tests/unit/test_bug780370.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,23 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* See https://bugzilla.mozilla.org/show_bug.cgi?id=780370 */
+
+const Cu = Components.utils;
+
+// Use a COW to expose a function from a standard prototype, and make sure it's
+// still remapped.
+
+function run_test()
+{
+  var sb = Cu.Sandbox("http://www.example.com");
+  sb.obj = { foo: 42, __exposedProps__: { hasOwnProperty: 'r' } };
+  do_check_eq(Cu.evalInSandbox('typeof obj.foo', sb), 'undefined', "COW works as expected");
+  do_check_true(Cu.evalInSandbox('obj.hasOwnProperty === Object.prototype.hasOwnProperty', sb),
+                "Remapping happens even when the property is explicitly exposed");
+  // NB: We used to test for the following, but such behavior became very
+  // difficult to implement in a recent refactor. We're moving away from this
+  // API anyway, so we decided to explicitly drop support for this.
+  // do_check_eq(Cu.evalInSandbox('Object.prototype.bar = 10; obj.bar', sb), 10);
+}