src/Pure/System/isabelle_tool.scala

--- a/src/Pure/System/isabelle_tool.scala	Tue Feb 28 17:51:49 2017 +0100
+++ b/src/Pure/System/isabelle_tool.scala	Tue Feb 28 19:50:08 2017 +0100
@@ -100,6 +100,7 @@
   private val internal_tools: List[Isabelle_Tool] =
     List(
       Build.isabelle_tool,
+      Build_Cygwin.isabelle_tool,
       Build_Doc.isabelle_tool,
       Build_Docker.isabelle_tool,
       Build_JDK.isabelle_tool,