gistfile1.scala

val fastOptJSCombine = taskKey[File]("Combine -fastopt.js with -launcher.js.")

fastOptJSCombine := {
  
  val fastOptFile = (fastOptJS in Compile).value.data
  
  val outputDir = fastOptFile.getParentFile
  val prefix    = name.value.toLowerCase + "-"
  
  val combinedFile  = outputDir / (prefix + "combined.js")
  val launcherFile  = outputDir / (prefix + "launcher.js")
  
  IO.delete(combinedFile)
  IO.append(combinedFile, IO.readBytes(fastOptFile))
  IO.append(combinedFile, IO.readBytes(launcherFile))
  
  combinedFile
}