[Null-aware elements] Language Tests in tests/language

[chloestefantsova]

Write up language tests for the tests/language repository.

The example is pulled from the spec.

void printThree(String? a, String? b, String? c) {
  print([?a, ?b, ?c].join(' '));
}

main() {
  printThree('first', null, 'last');
}

[chloestefantsova]

The current direction is to convert the emergent unit tests into language tests and to get @lrhn's help to generate particularly tricky examples.