Add a test where a class that has no generative constructors
declares an instance variable which is final and has no initializing
expression. With https://github.com/dart-lang/language/pull/1130 this
is an error, and that error is expected by the test. Also test that
it is not an error to have an uninitialized final variable which is
abstract or external.
Change-Id: I6c4a46393f46587448612bd91fb99fa65f89704c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/157490
Commit-Queue: Erik Ernst <eernst@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>