Change SharedRecordType abstract getters to return lists.

Previously these abstract getters were typed as returning
iterables. Although in principle, changing the return types to lists
constrains the implementations more, in practice it makes no
difference because the only implementations are in the CFE, analzyer,
and the mini_types shared tests, and those implementations already
return lists anyhow.

Changing the abstact getters to return lists will simplify future work
by allowing code that interacts with the base SharedRecordType class
to index into the lists rather than having to iterate through them.

Change-Id: Ia257400dbb7c89aabc468be15a939e6baa36e191
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/367500
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>