The issue is with the new optimizations for data reading. Internally, a generated function uses field names to hold values but if the field names are reserved JS keywords, you'll have this kind of error. This is a high priority bug for us to get fixed.
Turns out (after checking) that this bug is already fixed for the next beta build... it just didn't make the cutoff for beta 2.
Director of Engineering - Frameworks (Ext JS / Sencha Touch)