Fix tests under -Ydelambdafy:method

We don't currently run a nightly build using `-Ydelambdafy:method`, that's why some tests diverged.

For this PR, a `-Ydelambdafy:method` build is [running here](https://scala-webapps.epfl.ch/jenkins/view/All/job/scala-checkin-manual/1354/console)

After this PR is merged, I will add the `Ydelambdafy:method` flag to the [nightly genbcode build](https://scala-webapps.epfl.ch/jenkins/view/2.11.x/job/scala-nightly-genbcode-2.11.x/)