Part 1: review in #35 2. method should inheritdoc since doc is on the interface.
Part 1: #35 comment part of 6. adding comment about what array intersect is doing. might want an exception when attempted to update things not allowed.
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
convert createLink to addDefinition
docs fixes suggested by dawehner
Fix t() to $this->t()
some dawehner fixes
reapply review #15
some doc fixes
Part 2: 2301723 Added namespace in MenuParentFormSelector.php