I mean that of the built-in ECMA objects: String, Number, Object, Function, RegExp, Error, Boolean, etc..
From what I have found from extensive search on my drive is that it comes from the Flash installation as follows, though it may only be the syntax - highlighting and completion code.
Find your Adobe Flash installation, the following is for CS4, Flash Player 10.
c:\Program Files\Adobe\Adobe Flash CS4\Common\Configuration\ActionScript 3.0\FP10\playerglobal.swc
Open the above .swc in WinRAR or something equivalent, you will see an XML file that seems to server as a reference, and a SWF file "library.swf" which holds all the code.
Hope this helps.