Merge pull request #1723 from Gerharddc/patch-1

Stop Permissions being redefined and breaking Jest

lib/extensions.js

@@ -257,11 +257,13 @@ module.exports = function(realmConstructor) {
                 permissions: '__Permission[]'
             }
         });
+        if (!realmConstructor.Permissions) {
             Object.defineProperty(realmConstructor, 'Permissions', {
                 value: permissionsSchema,
                 configurable: false
             });
         }
+    }
 
     // TODO: Remove this now useless object.
     var types = Object.freeze({