When I try to save the home project of image 3.10.2-7179 (Damien Cassou's web based image), I get a DNU. Here is the last four context from the debugger:

Trait(Object)>>doesNotUnderstand: #isSystemDefined
    Receiver: TWriteStreamTest
    Arguments and temporary variables:
        aMessage:     isSystemDefined

This happens on both: VM versions 3.10.6 (2007) and 3.11.5 (Sep 2009).