[Declaring Constructors] Add co19 tests for the declaring constructors feature

[kallentu]

Spec: https://github.com/dart-lang/language/blob/main/accepted/future-releases/primary-constructors/feature-specification.md

This issue tracks the tests for primary constructors.
If there's already an issue, LMK!

cc. @sgrekhov @eernstg