Tests: Adding missing dependency

Author: tjvantoll

Diff for: tests/visual/compound/datepicker_dialog.html

@@ -12,6 +12,7 @@
 	<script src="../../../ui/position.js"></script>
 	<script src="../../../ui/resizable.js"></script>
 	<script src="../../../ui/draggable.js"></script>
+	<script src="../../../ui/button.js"></script>
 	<script src="../../../ui/datepicker.js"></script>
 	<script src="../../../ui/dialog.js"></script>
 	<script>