click file>publish settings>flash (swf)>and select actionscript 2.0 from the script combobox.
Perfect. That seems to have sorted it out. Do you know how it would have gotten changed? I'd like to make sure I don't do that again.
i've never seen that happen spontaneously so i suspect that file was saved as an as3 file at some time.
in any case, you can always change publish settings and fix any accidental changes.