It turns out that the has_member check is an important (accidental?)
performance optimization. Removing this caused a major (30+%?)
slowdown at dropbox. There might be a better way to optimize this but
I'm just going to revert it for now at least.

This reverts commit 09cdab4.

It is a pretty simple matter of pulling out the assignment target from
the walrus. We don't bother handling things like `x := (y := z)` since
I can't imagine they are common enough to be worth bothering but we
could in the future if anyone cares.

Fixes #8447.