Sorry to bump an old thread but I was looking for the answer to this same question and didnt find it in the forums. Then I managed to figure it out, so I thought it would be nice to post the answer here in case anyone else searches for it in the future.
I managed to figure this out by looking at the Ext build file (ext.jsb). It contains the following lines which tell jsbuilder to combine&minify source/core/Ext.js and source/adapter/ext-base.js into adapter/ext/ext-base.js. So including source/core/Ext.js and source/adapter/ext-base.js into your page will give you the unminified version of adapter/ext/ext-base.js
<target name="Ext Base" file="$output\adapter\ext\ext-base.js" debug="False" shorthand="False" shorthand-list="YAHOO.util.Dom.setStyle
<include name="core\Ext.js" />
<include name="adapter\ext-base.js" />