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

Fixes the `GenBCode` / `delambdafy:method` build (https://scala-webapps.epfl.ch/jenkins/view/2.11.x/job/scala-nightly-genbcode-2.11.x/191/console)