testAddCurrentDragItemClasses() {
    /** @suppress {checkTypes} suppression added to enable type checking */
    const be = new BrowserEvent({
      type: EventType.MOUSEDOWN,
      button: 0,
    });
    /**
     * @suppress {visibility,checkTypes} suppression added to enable type
     * checking
     */
    event.event_ = be;

    dlg.handlePotentialDragStart_(event);

    assertFalse(
        'Should have no cursor_move class after MOUSEDOWN',
        classlist.contains(dlg.currDragItem_, 'cursor_move'));
    assertFalse(
        'Should have no cursor_pointer class after MOUSEDOWN',
        classlist.contains(dlg.currDragItem_, 'cursor_pointer'));
    assertTrue(
        'Should have opacity_40 class after MOUSEDOWN',
        classlist.contains(dlg.currDragItem_, 'opacity_40'));
    assertTrue(
        'Should have blue_bg class after MOUSEDOWN',
        classlist.contains(dlg.currDragItem_, 'blue_bg'));
  }
Test the effect of dragging an item. (DRAGSTART event.)
Check that after the MOUSEDOWN browser event is handled by the
handlePotentialDragStart_() method the currDragItem has the CSS classes
set by the setter method.
@suppress {visibility} suppression added to enable type checking