* a11y: implement new SemanticsAction "showOnScreen" (v2)
This action is triggered when the user swipes (in accessibility mode) to the last visible item of a scrollable list to bring that item fully on screen.
This requires engine rolled to flutter/engine#3856.
I am in the process of adding tests, but I'd like to get early feedback to see if this approach is OK.
* fix null check
* review comments
* review comments
* Add test
* fix analyzer warning
b5c461a917