Analyzer doesn’t flush errors for files removed from priorityFiles that aren’t otherwise analyzed

As mentioned here:

#25551 (comment)

If you send a priority file that is not otherwise analyzed, the server analyzes it, but doesn’t flush its errors when it’s removed from priority files. This causes the errors to hang around (I would expect until the IDE restarts, however due to caching maybe it’s forever unless you clear ~./dartServer based on Dart-Code/Dart-Code#1480 (comment)?).