ce81216885
This is in preparation to migrate package:kernel to null safety. For the visitor interfaces to support non-nullable return types, the implementations must avoid using `null` as return value in its base case. TEST=Refactoring Change-Id: Ie5e4153f8d3779d94957bb13b3d2d2a942040ff2 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/179760 Commit-Queue: Johnni Winther <johnniwinther@google.com> Reviewed-by: Jens Johansen <jensj@google.com>