address a valid unchecked warning
address a valid unchecked warning

In order to correctly pattern match Tree subclasses in reflection/macros,
scalac needs the corresponding implicit for *Tag available because the types
are only abstract types.

xeno-by/old-scalameta-sbt,Duhemm/compiler-bridge,jaceklaskowski/sbt,sbt/sbt,corespring/sbt,harrah/xsbt,twitter-forks/sbt,jkinkead/sbt,jkinkead/sbt,twitter-forks/sbt,mdedetrich/sbt,xuwei-k/xsbt,harrah/xsbt,sbt/sbt,twitter-forks/sbt,mdedetrich/sbt,sbt/sbt,dyx/sbt,xuwei-k/xsbt,Sciss/sbt,jkinkead/sbt,harrah/xsbt,jaceklaskowski/sbt,jasonchaffee/sbt,xuwei-k/xsbt,jkinkead/sbt,harrah/xsbt,sbt/sbt,adriaanm/sbt,xeno-by/old-scalameta-sbt,jaceklaskowski/sbt,sbt/sbt,harrah/xsbt,xuwei-k/xsbt,mdedetrich/sbt,olove/xsbt,xeno-by/old-scalameta-sbt,twitter-forks/sbt,Duhemm/sbt,dyx/sbt,jaceklaskowski/sbt,xeno-by/old-scalameta-sbt,dyx/sbt,mdedetrich/sbt,sbt/sbt,dyx/sbt,pdalpra/sbt,jkinkead/sbt,sbt/sbt,xeno-by/old-scalameta-sbt,dansanduleac/sbt,dyx/sbt,jaceklaskowski/sbt,mdedetrich/sbt,twitter-forks/sbt,olove/xsbt,olove/xsbt,olove/xsbt,mdedetrich/sbt,som-snytt/xsbt,harrah/xsbt,xeno-by/old-scalameta-sbt,mdedetrich/sbt,Duhemm/sbt,xuwei-k/xsbt,pocketberserker/sbt