Add helper on DartType to compute the `NonNull` of a type and use this
instead of `withDeclaredNullability(Nullability.nonNullable).
Includes a fix in the computation of NonNull of FutureOr.
TEST=existing
Change-Id: I3399cbf89c7d3f3e90f1315b01f40957e798a1b1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193400
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Johnni Winther
13556d623f