Domains dispose problem

Problem: When creating streams inside of a domain, Domain#dispose() will
not clean them up.

This patch demonstrates this problem using a test case, and provides a
possible fix by also adding implicit domain members to the domain.

Considering that I don't understand the design decisions that went into
only tracking explicit members at this point, this patch should be
considered as a request for comment on the current behavior / to start a
discussion on improving it.