add support for experimental private abstract references syntax - closes #291

lib/6to5/patch.js

@@ -30,6 +30,11 @@ def("VirtualPropertyExpression")
   .field("object", def("Expression"))
   .field("property", or(def("Identifier"), def("Expression")));
 
+def("PrivateDeclaration")
+  .bases("Declaration")
+  .build("declarations")
+  .field("declarations", [def("Identifier")]);
+
 // Playground
 def("BindMemberExpression")
   .bases("Expression")