Try using CSS display:none
to hide it :
<a id="done-modal-button" ng-click='onCompleteTask(todo)' ng-model="todo.done" class="btn btn-xs btn-success" style="display:none;">Done</a>
Update based on your comment
To toggle visibility depending on weather a checkbox is checked or not you need to see if it's checked first, and if it is, show the button, else hide it back.
$(document).ready(function() {
$('#checkboxid').click(function() {
if($(this).is(":checked"))
{
$('#done-modal-button').show();
} else {
$('#done-modal-button').hide();
}
});
});
Working JSfiddle
Update 2
I guess you have no other jquery library in your script, to use the method above you could use the method in the jsfiddle I gave you in the comment OR you could load jquery-1.9.1 and it will work, see this JSFIDDLE with AngularJS and jQuery-1.9.1
To load the library just add this inside your head tags:
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>