When writing action script using code hinting, Flash adds additional code at line 1, such as:
import flash.events.MouseEvent;
What is that line for? I've noticed the swf runs fine without it. How important is it that this line exist?
it explicitly imports a class needed by your swf. in a fla, that doesn't need to exist. flash will import the needed class when you publish the swf. it will just be done implicitly.
if you create a class file, that line will need to be present because needed classes are only imported if explicitly coded.