Part 1: 2301239 cleaning up MenuLinkContent.php.
Part 1: 2301239 cleaned up class in MenuLinkContent.php
code comment fixes
return doc fix
document what NULL means
document return values for menu tree storage
Part 1: 2301239 Cleaned up class in MenuLinkContent.php and added type hint in MenuLinkContentInterface.
add an exception if you try to re-create a plugin
add an exception if you try to create an invalid plugin
doxygen fixes
START PART 2:  https://www.drupal.org/node/2301273
apply patch https://www.drupal.org/files/issues/menu-2256521-MenuLinkTree-review-do-not-test.patch
START PART 3: https://www.drupal.org/node/2301313
apply patch https://www.drupal.org/files/issues/menu-2256521-MenuLinkContentUI-review-do-not-test.patch