fix incorrect getThisReference bind - fixes #688

lib/6to5/transformation/helpers/replace-supers.js

@@ -150,7 +150,7 @@ var visitor = {
       // top level so `this` is the instance
       t.thisExpression :
       // not in the top level so we need to create a reference
-      self.getThisReference;
+      self.getThisReference.bind(self);
 
     var callback = self.specHandle;
     if (self.isLoose) callback = self.looseHandle;