src/Pure/Tools/dotnet_setup.scala

     for {
       platform <- all_platforms
       if platform.family.toString == platform_spec || platform.name == platform_spec
     } {
       progress.expose_interrupt()
-      platform.check()
 
 
       /* component directory */
 
       val component_dir =

         if (platform_dir.is_dir) {
           progress.echo_warning("Platform " + platform.name + " already installed")
         }
         else {
           progress.echo("Platform " + platform.name + " ...")
+          platform.check()
           val script =
             platform.exec + " " + File.bash_platform_path(install) +
               (if (version.nonEmpty) " -Version " + Bash.string(version) else "") +
               " -Architecture " + Bash.string(platform.arch) +
               (if (platform.os.nonEmpty) " -OS " + Bash.string(platform.os) else "") +