[Dart] Modal (Popup)

<!-- Button trigger modal -->
<button type="button" data-toggle="modal" data-target="#myModal">
  Launch demo modal
</button>

<!-- Modal -->
<div class="modal" id="myModal">
  <h4>Modal Title</h4>
  <p>
     Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas turpis
     felis, tempus sit amet sollicitudin quis, aliquet ut felis.
     Nam a malesuada sem.
  </p>
  <button type="button" data-dismiss="modal">Close</button>
</div>

.modal {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translateX(-50%) translateY(-50%);
  background: azure;
  padding: 1rem;
  border-radius: 1rem;
  border: 1px solid gray;
  display: none;
  color: black;
}

import 'dart:html';

void launchModal(Event e) {
  // when user click launch button, show modal.
  ButtonElement elm = e.target;
  var selector = elm.dataset["target"];
  var modal = querySelector(selector);
  modal.style.display = "block";

  if (modal.dataset["isSetClose"] != "true") {
    // when user click button with `data-dismiss="modal"` in the modal,
    // close the modal
    var closeBtns = modal.querySelectorAll("button[data-dismiss='modal']");
    for (var closeBtn in closeBtns) {
      closeBtn.onClick.listen((e) => modal.style.display = "none");
    }

    modal.dataset["isSetClose"] = "true";
  }
}

void main() {
  var launchBtns = querySelectorAll("button[data-toggle='modal']");

  for (var launchBtn in launchBtns) {
    launchBtn.onClick.listen(launchModal);
  }
}