Make t8960 pass under delambdafy:method

When using delambdafy:method, anonymous function classes are not
currently specialized, as noted here:

https://github.com/scala/scala/blob/f08e96571479552b103b15cc2d40ea5454999546/src/compiler/scala/tools/nsc/transform/Delambdafy.scala#L26
Merge pull request #4110 from lrytz/t8960-delambdafy

Make t8960 pass under delambdafy:method