Ok, I got that myself.

I deleted all DLLs out of bin folder, downloaded Umbraco 7.2.2 sources, added them to my VS solution to understand how everything works. Turns out there is Umbraco.Web\Trees\ContentTreeController.cs:

protected override TreeNode GetSingleTreeNode(IUmbracoEntity e, string parentId, FormDataCollection queryStrings)
{
    var entity = (UmbracoEntity) e;

    var allowedUserOptions = GetAllowedUserMenuItemsForNode(e);
    if (CanUserAccessNode(e, allowedUserOptions))
    {

        //Special check to see if it ia a container, if so then we'll hide children.
        var isContainer = e.IsContainer();   // && (queryStrings.Get("isDialog") != "true");

        var node = CreateTreeNode(
            e.Id.ToInvariantString(),
            parentId,
            queryStrings,
            e.Name,
            entity.ContentTypeIcon,
            entity.HasChildren && (isContainer == false));

and

Umbraco.Core\Models\UmbracoEntityExtensions.cs:

internal static class UmbracoEntityExtensions
{

    // ...

    public static bool IsContainer(this IUmbracoEntity entity)
    {
        if (entity.AdditionalData.ContainsKeyIgnoreCase("IsContainer") == false) return false;
        var val = entity.AdditionalData.GetValueIgnoreCase("IsContainer", null);
        if (val is bool && (bool) val)
        {
            return true;
        }
        return false;
    }

}

so I did the same thing, TLayout.cshtml:

                    var menuItems = homePage.Children;

                    // ...

                    IUmbracoEntity entity = ApplicationContext.Services.EntityService.Get(item.Id, UmbracoObjectTypes.Document);

                    object val = null;
                    entity.AdditionalData.TryGetValue("IsContainer", out val);

                    bool isContainer = val is bool && (bool)val;


                    if (item.Children.Count() > 0 && !isContainer)
                    {
                        // Display item and every child
                    }
                    else
                    {
                        // Display item only, just like in Content panel of Umbraco backoffice
                    }

Profit!