I frequently write ActionScript in ST2 and Toggle Comment works fine for me, so I thought this was either a Yosemite bug or an isolated issue. But I just tested in a fresh install of ST3 on XP, and lo and behold, I get the exact behaviour you describe. Digging around in my ST2 customisations (and memory), I now recall that ActionScript comments have always been broken. Sorry, apparently I fixed this long ago on my machine but had forgotten that.
Anyway here’s a fix. Paste the following into a “Comments.tmPreferences” file and place it in a “/Data/Packages/User/ActionScript” directory.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>name</key>
<string>Comments</string>
<key>scope</key>
<string>source.actionscript</string>
<key>settings</key>
<dict>
<key>shellVariables</key>
<array>
<dict>
<key>name</key>
<string>TM_COMMENT_START</string>
<key>value</key>
<string>//</string>
</dict>
<dict>
<key>name</key>
<string>TM_COMMENT_START_2</string>
<key>value</key>
<string>/*</string>
</dict>
<dict>
<key>name</key>
<string>TM_COMMENT_END_2</string>
<key>value</key>
<string>*/</string>
</dict>
</array>
</dict>
<key>uuid</key>
<string>6E85C1ED-6F2D-443B-B326-64947FE44621</string>
</dict>
</plist>
On my XP system this works for both ST2 and 3. I’ve generated a new UUID key for you in the above code, but you can create your own at this site, if you prefer:
famkruithof.net/uuid/uuidgen
Hope that fixes it for you!