[dart migrate] Can't disable a directory with interactive html tool

[esDotDev]

I'm using dart migrate, and am inside the HTML suggestions page. According to the docs, you can disable a package with the checkbox, but it's not working.
". To opt out a file or directory, click its green checkbox"

I can disable individual files, but not an entire folder, I get this error:

Uncaught TypeError: Cannot read property 'querySelector' of null
    at Object.hl (flutter-app?authToken=GvgdqvwceKs%3D:3464)
    at Object.hl (flutter-app?authToken=GvgdqvwceKs%3D:3465)
    at TD.$1 (flutter-app?authToken=GvgdqvwceKs%3D:7015)
    at vN.$1 (flutter-app?authToken=GvgdqvwceKs%3D:6407)
    at ft (flutter-app?authToken=GvgdqvwceKs%3D:388)
    at HTMLSpanElement.<anonymous> (flutter-app?authToken=GvgdqvwceKs%3D:396)

[stereotype441]

I spent some time manually de-obfuscating the stack trace. Here's the code in question, from pkg/nnbd_migration/lib/src/front_end/web/migration.dart:

void updateSubtreeIcons(Element element, NavigationTreeDirectoryNode entity) {
  for (var child in entity.subtree) {
    var childNode = element.querySelector('[data-name*="${child.path}"]'); // (1)
    if (child is NavigationTreeDirectoryNode) {
      updateSubtreeIcons(childNode, child); // (2)
      var childIcon = childNode.querySelector(':scope > .status-icon');
      updateIconsForNode(childIcon, entity);
    } else {
      var childIcon = (childNode.parentNode as Element)
          .querySelector(':scope > .status-icon');
      updateIconsForNode(childIcon, child);
    }
  }
}

The top stack entry is at (1) and the next stack entry is at (2). The third stack entry is here (inside the method writeNavigationSubtree):

        statusIcon.onClick.listen((MouseEvent event) {
          toggleDirectoryMigrationStatus(entity);
          updateSubtreeIcons(li, entity); // (3)
          updateIconsForNode(statusIcon, entity);
          updateParentIcons(li, entity);
        });

So I think what has happened is that when the user clicked on the status icon, updateSubtreeIcons tried to use element.querySelector('[data-name*="${child.path}"]') to read the childNode, and got back null. Since the child was a NavigationTreeDirectoryNode, it made a recursive call to itself (passing null for element), and that in turn caused the null reference exception at (1).

So why did the call to element.querySelector return null? I'm guessing the problem is that since @esDotDev is using Windows (I can tell from the C: in their screenshot), the string passed to querySelector contained backslashes. Backslashes passed to querySelector need to be escaped (see https://developer.mozilla.org/en-US/docs/Web/API/Document/querySelector#escaping_special_characters).

I'm working on a fix now.