I downloaded the as3corelib.swc being used by that project, and ran the following code.
SHA1.hashToBase64('k9NwkeRGtpHKwr+kcSk29tHaAbs=');
In addition to your output, which basically says Flash Player threw an error trying to construct a new Base64Encoder
object, I got the following output.
VerifyError: Error #1014: Class mx.utils::Base64Encoder could not be found.
It seems to be a known issue with as3corelib that was never resolved.
However, this dependency is also open source, so I just downloaded Base64Encoder.as, and added it to my project. After this, the code compiled fine. This should work for your project, as well.