less eager with the sam

before, bootstrap fails:
[quick.library] /localhome/jenkins/b/workspace/scala-checkin-manual/src/library/scala/collection/parallel/ParIterableLike.scala:736: error: recursive method bf2seq needs result type
[quick.library]   def scanLeft[S, That](z: S)(op: (S, T) => S)(implicit bf: CanBuildFrom[Repr, S, That]) = setTaskSupport(seq.scanLeft(z)(op)(bf2seq(bf)), tasksupport)
[quick.library]                                                                                                                               ^