DropDownList parameter cannot be made 'required'

An HST component parameter annoted with DropDownList cannot be made 'required' in the UI.

Steps to reproduce:
1. add a required component parameter with DropDownList annotation. For example, in the HAP Christmas Tree component, use:

@Parameter(name = "dropdown", required = true)
@DropDownList(value = {"one", "two", "three"})
String getDropDown();

2. drag the component into a container in a channel
3. edit the component

Expected: the 'dropdown' parameter is marked with an asterisk, and the component cannot be saved without selecting a value from the dropdown.
Actual: the 'dropdown' parameter is not marked with an asterisk, and the component can be saved without selecting any value from the dropdown.

This issue has been reported on the community list at 2015-12-28.