Step 3: nits in comments in MenuLinkContentForm. Capitalizing some things.
Part 3: typo route_nae
START PART 4: https://www.drupal.org/node/2301317
apply patch https://www.drupal.org/files/issues/menu-2256521-conversion-review-do-not-test.patch
and resolve 8.x conflicts
fix hook_uninstall to use the correct service
rename updateLink to updateDefinition
rename deleteLink to removeDefinition
rename isResetable() to isResettable()
convert topLevelOnly() to setTopLevelOnly()
fix destination
another destination fix
partial test fix
rename yml files
add back system block caching
don't eanble menu_link module per Alex
move SafeMarkup fix to part4
reference https://www.drupal.org/node/2306991