Don't try to be smart with `unsupportedMembers`

[copybara-service[bot]]

Don't try to be smart with unsupportedMembers

If MockSpec says a member is not supported, just generate a throw, even if it looks like we could get away without it.

[yanok]

Uh.. I'm not sure it was supposed to appear here: still not submitted internally.

[srawlins]

Haha maybe just a PR to check external CI (like issues with pub package dependencies). Glad I didn't click 'Merge' 😁

[yanok]

Oh yes, I added that Copybara presubmit recently. But I didn't realize it's going to be a public PR for every CL I try ;) Going to be fun :)