It appears that calling methods on element variables directly from a datasource extension does not work with Chain of Command. We had this problem before U8 on form extensions as well.

A workaround is to create a local variable and reference the element variable.

More information: I noticed that when adding lines of code after the problematic line, it causes more compiler errors, which has me thinking this might be a parsing/interpretation related error.

Reproduction:
1. Create a formDataSource extension
2. Wrap an existing datasource method using CoC
3. Access a method on a form control, such as allowEdit()

More info:
https://www.yammer.com/dynamicsaxfeedbackprograms/uploaded_files/155158828
and
https://www.yammer.com/dynamicsaxfeedbackprograms/threads/1175297249