On JDK 1.9 the build of some modules fails with:
error: scala.reflect.internal.MissingRequirementError: object java.lang.Object in compiler mirror not found.
at scala.reflect.internal.MissingRequirementError$.signal(MissingRequirementError.scala:17)
at scala.reflect.internal.MissingRequirementError$.notFound(MissingRequirementError.scala:18)
at scala.reflect.internal.Mirrors$RootsBase.getModuleOrClass(Mirrors.scala:53)
at scala.reflect.internal.Mirrors$RootsBase.getModuleOrClass(Mirrors.scala:45)
at scala.reflect.internal.Mirrors$RootsBase.getModuleOrClass(Mirrors.scala:45)
at scala.reflect.internal.Mirrors$RootsBase.getModuleOrClass(Mirrors.scala:66)
at scala.reflect.internal.Mirrors$RootsBase.getClassByName(Mirrors.scala:102)
at scala.reflect.internal.Mirrors$RootsBase.getRequiredClass(Mirrors.scala:105)
at scala.reflect.internal.Definitions$DefinitionsClass.ObjectClass$lzycompute(Definitions.scala:257)
at scala.reflect.internal.Definitions$DefinitionsClass.ObjectClass(Definitions.scala:257)
at scala.reflect.internal.Definitions$DefinitionsClass.init(Definitions.scala:1387)
at scala.tools.nsc.Global$Run.<init>(Global.scala:1053)
at scala.tools.nsc.Driver.doCompile(Driver.scala:31)
at scala.tools.nsc.MainClass.doCompile(Main.scala:23)
at scala.tools.nsc.Driver.process(Driver.scala:51)
at scala.tools.nsc.Driver.main(Driver.scala:64)
at scala.tools.nsc.Main.main(Main.scala)

The underlying problem is https://issues.scala-lang.org/browse/SI-9103 which means that Scala does not work on JDK 1.9 yet.