Part 4: nits as was reading the patch. Period at end of sentence in a changed line and taking out an accidental blank line that was added.
Part 4: removing an unused use in RouterRebuildSubscriber that was added in this patch. Maybe it was needed earler and now is not.
Part 4: nit. wrapping comment to 80 chars.
Part 4: doTestmenuLinksDiscoveredAlter could use a close look. Updated names since hook_menu_link_defaults_alter was replaced with hook_menu_links_discovered_alter.
getTitle and getDescription should also be abstract in the base class
discovered is not part of the defintion now
fix ternary special case for front page in LinkGenerator
fix title to description in ViewsMenuLinkForm
Part 4: very nit to alpha order use addition.
Part 4: a use was added, but now it is not needed anymore. taking it out of menu_ui.admin.inc
Part 4: putting a use that was added in alpha order.
Part 4: taking out some unrelated clean up in uses that were already existing.
START PART 5: https://www.drupal.org/node/2301319
apply patch https://www.drupal.org/files/issues/menu-2256521-remove-dead-code.patch
and resolve conflicts with 8.x
remove unused constant MENU_MAX_DEPTH