Step 3: Add punctuation to sentence in MenuLinkEditForm.
Step 3: methods not inherited should not have inheritdoc. Made up docs. Form docs.
remove stray methods that aren't actually part of MenuLinkInterface
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
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