[ddc] Fix variance test migration to null safety
[ddc] Fix variance test migration to null safety

Change-Id: I5f6e446c72ae1a41d92f419eb57a8dd030a70372
Issue: https://github.com/dart-lang/sdk/issues/42107
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154292
Reviewed-by: Leaf Petersen <9bef692239b5f624cec8e2cd5249ab1939a377aa@google.com>
Commit-Queue: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@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