Add definite assignment tests.

New tests for the definite assignment related changes specified in
dart-lang/language#1091 .

Change-Id: I119e36c1bddca97f7021c9015b0f2eee5cb80969
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/157881
Reviewed-by: Erik Ernst <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: leafpetersen