If you are not using jQuery (which you don't seem to), you should do the instantiation/binding and the appending in different statements:
var resizer = angular.element('<div class="myResizable__resizer"></div>')
.bind('mousedown', function ($event) {...});
elm.append(resizer);
If you were using jQuery, you could use the appendTo()
function (which is not implemented in JQLite), you could do it like this:
$('<div class="myResizable__resizer"></div>').appendTo(elm).bind(...);