My macbook thinks .as files are zip files (and they won't open in XCode)
Xcode doesn't support ActionScript files out of the box, so it sees your file and thinks it's an AppleSingle file instead, as does the rest of your Mac, since that's how OS X usually understands files with the .as extension by default.

Unfortunately I couldn't find any information on getting Xcode to support ActionScript projects. There is this Stack Overflow question, but it (and the resources the answers point to) doesn't give you much to go on, and I can't really vouch for any of the answers either since I don't do AS development on my Mac.

TextEdit should open your script in plaintext mode with no issues, though. If you just want to give it a quick read, you might just have to go with TextEdit for now until you can acquire Flash or another IDE.

The reason why it used to work on your old Mac, presumably, is because you had Flash or a different IDE that designated .as files as ActionScript files, overriding the system default of AppleSingle file. Based on this, installing Flash should be enough to get your Mac to recognize .as files as ActionScript files.