[ddc] Fix crash on unspecified nullability
[ddc] Fix crash on unspecified nullability

When encountering a FutureOr<T> where both the type argument and
FutureOr have unspecified nullability, compile the FutureOr type
without any nullability wrappers.

Change-Id: Ic822e9d5cb74864a424963d5e624c4e2bf311b33
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/203122
Commit-Queue: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>
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