[ddc] Add unspecified nullability regression test
[ddc] Add unspecified nullability regression test

Change-Id: If82ef47aac1d67c5f6db636522ed0ef7de180f8d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/203121
Reviewed-by: Mark Zhou <654f5fe0c4ca6c799b0e735dcf3fc1c389f35bf9@google.com>
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk