mx.util Base64Encoder cannot be found Flash CS5

In my main fla file I've linked to:

C:\Program Files (x86)\Adobe\Adobe Flash Builder
4.5\sdks\4.5.0\apache_flex\frameworks\libs\mx\mx.swc

In File / Actionscript Settings / Library Path

But I still get errors for Base64Encoder in my .as class script:

C:\Users\User\Desktop\web-socket-js-master\flash-src\src\net\gimite\websocket\WebSocket.as,
Line 75 1046: Type was not found or was not a compile-time constant:
Base64Encoder.

C:\Users\User\Desktop\web-socket-js-master\flash-src\src\net\gimite\websocket\WebSocket.as,
Line 75 1180: Call to a possibly undefined method Base64Encoder.

C:\Users\User\Desktop\web-socket-js-master\flash-src\src\net\gimite\websocket\WebSocket.as,
Line 75 1180: Call to a possibly undefined method Base64Encoder.

The docs for Flex Apache show Base64Encoder to be in mx.utils but If I also
try

import mx.utils. the only code hints are LoaderUtil, NameUtil, StringUtil

Why?