`generateLocalizations` should fail (`flutter gen-l10n`) if `flutter:
generate:` does not exist.
The previous logic was faulty, because it was totally possible to
opt-out of synthetic packages (i.e. in a `l10n.yaml` file), but still
not be specifying `flutter: generate:`, which I _believe_ is supposed to
still be an error.
This came up in https://github.com/flutter/flutter/pull/160289 as
`flutter config --explicit-package-dependencies` is enabled by default,
as the error is no longer thrown. Made a few other small test
forward-fixes that otherwise would break with the switch (but are
expected) as well.
1e32c00349