added test for timestamp constant
added test for timestamp constant

collinjackson/firebase2-dart,kevmoo/firebase-dart,collinjackson/firebase2-dart,sirctseb/firebase-dart,kevmoo/firebase-dart,sirctseb/firebase-dart,collinjackson/firebase2-dart,collinjackson/firebase2-dart,sirctseb/firebase-dart,kevmoo/firebase-dart