I don't know how (in Javascript) eval and return interact. It might be worth investigating if the problem comes up again in a different place.

So far, there is only one piece in MozRepl::RemoteObject needing functions, the implementation of ->declare(), and that one was easily patched to support the temporary assignment to a variable. Changing the Javascript code to become eval "return $expr" does not work in the general case because $expr is allowed to contain multiple statements. I don't want to implement a JS parser just to find the last statement in some block and put the return before it, so I just change the call sites of that code instead.