Simplify the checkAccess() tree manipulator.
Step 1 in actually using DefaultMenuTreeManipulators: remove getChildLinks(), convert to buildSubTree().
Step 2 in actually using DefaultMenuTreeManipulators: make buildSubtree() build a tree, not a tree node, to allow tree manipulators to be applied; removed access checking and tree sorting from MenuLinkTree in favor of the manipulators that perform those tasks.
s/DefaultMenuTreeManipulators/DefaultMenuLinkTreeManipulators/
DefaultMenuLinkTreeManipulators doesn't use the route provider service.
Ported the DefaultMenuLinkTreeManipulators test coverage; fixed a bug thanks to it.
Introduce MenuLinkTree::transform().
Remove the calls to MenuLinkTree::transform() in MenuLinkTree::build() and MenuLinkTree::buildSubTree(). Move the responsibility to the caller of ::build(SubTree()) instead.
Convert toolbar_menu_navigation_links() to a menu tree manipulator.
Inject MenuLinkTree into SystemController rather than using \Drupal::menuTree().
Cleaner way of getting at the actual subtree; removed one last direct call to DefaultMenuLinkTreeParameters.