Fix #718.

lib/6to5/transformation/templates/tail-call.js

@@ -5,6 +5,8 @@
     this.context = context;
   }
 
+  Tail.prototype._isTailDescriptor = true;
+
   var isRunning = false;
 
   return function (func, args, context) {
@@ -13,7 +15,7 @@
       isRunning = true;
       do {
         result = result.func.apply(result.context, result.args);
-      } while (result instanceof Tail);
+      } while (result instanceof Tail || (result && result._isTailDescriptor));
       isRunning = false;
     }
     return result;