Build devicelab iOS apps as universal binaries - #18102

How are we going to automatically update everybody's apps?

flutter create has been generating universal binaries by default since 32-bit support landed. In terms of upgrading existing apps,

There are few couple avenues for upgrading existing apps:

1. Email to flutter-dev: there are instructions for manually updating in the Wiki.
2. We don't yet have anything like a flutter doctor for projects, but we could emit an info-level log message pointing people at the wiki if we spot a 64-bit only app. In rare cases, this may be intentional though, which could potentially get annoying.
3. Not really a strategy, but with every new Xcode release, any non-default settings generate a warning and Xcode offers to reset those settings to default; the architecture is one of those settings. I'd expect some fraction of users to upgrade organically.

Seems like a good fit for #12573.

Anyway, LGTM, I guess. but I really think we should get to a point soon where we never make changes to the templates unless we can autoupgrade existing packages. We're not testing that the unchanged templates continue to work, which means essentially that our customers are getting further and further away from what we actually test (since nobody is actually upgrading when we send out instructions to upgrade).

SGTM. Added a comment to #12573 that we'll probably want project upgrades to support step-by-step confirmation, since there are cases where an app may legitimately be 64-bit only and we'd want the user to have to option to not upgrade.