Stephen Adams
770969e25c
`BaseVisitor<R>` has two methods that return `null`, which will not match the generic return type with NNBD. The fix is to make the methods abstract, and provide a convenience specialization `BaseVisitorVoid` with the default methods. Change-Id: I62cd69c5ed1c912b2804da5044e2cda93d70b878 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/224460 Reviewed-by: Nicholas Shahan <nshahan@google.com> Commit-Queue: Stephen Adams <sra@google.com>