diff -r 000000000000 -r 6474c204b198 services/sync/tests/unit/test_utils_deferGetSet.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/services/sync/tests/unit/test_utils_deferGetSet.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,45 @@
+_("Make sure various combinations of deferGetSet arguments correctly defer getting/setting properties to another object");
+Cu.import("resource://services-sync/util.js");
+
+function run_test() {
+  let base = function() {};
+  base.prototype = {
+    dst: {},
+
+    get a() "a",
+    set b(val) this.dst.b = val + "!!!"
+  };
+  let src = new base();
+
+  _("get/set a single property");
+  Utils.deferGetSet(base, "dst", "foo");
+  src.foo = "bar";
+  do_check_eq(src.dst.foo, "bar");
+  do_check_eq(src.foo, "bar");
+
+  _("editing the target also updates the source");
+  src.dst.foo = "baz";
+  do_check_eq(src.dst.foo, "baz");
+  do_check_eq(src.foo, "baz");
+
+  _("handle multiple properties");
+  Utils.deferGetSet(base, "dst", ["p1", "p2"]);
+  src.p1 = "v1";
+  src.p2 = "v2";
+  do_check_eq(src.p1, "v1");
+  do_check_eq(src.dst.p1, "v1");
+  do_check_eq(src.p2, "v2");
+  do_check_eq(src.dst.p2, "v2");
+
+  _("make sure existing getter keeps its functionality");
+  Utils.deferGetSet(base, "dst", "a");
+  src.a = "not a";
+  do_check_eq(src.dst.a, "not a");
+  do_check_eq(src.a, "a");
+
+  _("make sure existing setter keeps its functionality");
+  Utils.deferGetSet(base, "dst", "b");
+  src.b = "b";
+  do_check_eq(src.dst.b, "b!!!");
+  do_check_eq(src.b, "b!!!");
+}