Contains both migrated and migrated-to properties

This is necessary because if collect_type_info processes a
migrated-to property first, and then processes a migrated property
later, PropInfo.migration will still be None and we won't be able to
apply migrations

This would not work if a logical property had multiple, different
migration operations associated with it, but thankfully that is not
the case!

This is kind of ugly...

Because migrated and migrated-to properties can both exist in the same
DOM, we're won't necessarily get a consistent variant type here, so we just ignore
any migration failures and assume values that fail are already the
correct type

Woo, lookin good!