Add FutureOrType to package:kernel

[johnniwinther]

[johnniwinther]

Currently FutureOr is encoded as an InterfaceType. This means that the DartType.nullability doesn't not hold the semantic but rather the declared nullability for FutureOr types:

Both FutureOr<int?> and FutureOr<int> have nullability Nullability.nonNullable but Null is semantically included in the former. Currently computeNullabilityOfFutureOr located in pkg/kernel/lib/src/future_or.dart is used to compute the semantic nullability of FutureOr types.