to be honest I am not sure why it is used in the file but it is something related to action script 3. I started to convert action script 3 to html 5 and as you said it was used in as files.
The case is that, if we are able to import flash.* components then why program does not see inside of the flash folder?
I realized that inside of the classes Path (Openfl) there is not anything related accessibility package. And what I did is :
I opened the class path folder ( in my case : C:\HaxeToolkit\haxe\lib\openfl\8,9,5\src\openfl) and added Accessibility folder from Flash (copied here). Then IDE occured Invalid Error, and I edited the AccessibilityProperties.hx and changed first line from package flash.accessibility; into package openfl.accessibility;
and wolaa who has this kind of issue try to do this. I am not sure if this will work at the end but at least I stopped to get an errors so far. anyone who has advanced level can suggest me if I did right or not.