This package is on SqueakMap:

http://map2.squeakfoundation.org/sm/package/9f2cdda3-c666-459c-ba5e-55a9
1b428f40

This packages was called RuntimeEnvironments.  The name change reflects
a change in the implementation which no longer uses an instance variable
in Process to hold the dynamic variable bindings.  Instead, lookup of
the active variable bindings is done by looking up the activation stack
(using the exception handling system).