Abstract field support: suppress "final not initialized" errors.

Abstract final fields don't need to be initialized (in fact they can't
be initialized), so we shouldn't report "uninitialized field" errors
for them.

This CL also adds some basic tests of the functionality of abstract
final fields, making sure they don't introduce setters, and that the
getters they introduce are properly resolved and that their types are
properly respected by inheritance checks.

Change-Id: I885f23ce7ca554ba8f48e932ba9d76e128ac0bbd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/157520
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>