Bug 568707 - "key is not defined" results in broken form history service [r=mconnor]

Pretend to call a function with "key" to keep it around.

services/sync/FormNotifier.js

@@ -24,6 +24,9 @@ function FormNotifier() {
       continue;
     }
 
+    // XXX Bug 568707 Make use of "key" to prevent it from disappearing
+    (function(){})(key);
+
     // Wrap the function with notifications
     this[key] = function() {
       let args = Array.slice(arguments);