fwt::Text output
...
output.text = output.text + "Done\n" 
output.text += "Not Done\n"  

Will be compiled into

output.text$(fan.sys.Str.plus(output.text(),"Done\n"));
output.text() = fan.sys.Str.plus(output.text(),"Not Done\n");

resulting in sys::Err: invalid assignment left-hand side

Promoted to ticket #1387 and assigned to andy

Yeah I know about that one - but lets open a ticket to track.

Ticket cancelled

This looks like a dup of #1683 - I'll track that one since it has a simpler test case.