fix lint errors

lib/6to5/transform.js

@@ -15,8 +15,10 @@ var transform = module.exports = function (code, opts) {
     format:    {}
   });
 
+  var tree;
+
   try {
-    var tree = util.parse(code);
+    tree = util.parse(code);
   } catch (err) {
     err.message = opts.filename + ": " + err.message;
     throw err;