simplify transform call chain

lib/6to5/file.js

@@ -84,7 +84,8 @@ File.prototype.parse = function (code) {
   code = this.parseShebang(code);
 
   return util.parse(this.opts, code, function (tree) {
-    return self.transform(tree);
+    self.transform(tree);
+    return self.generate();
   });
 };
 
@@ -96,8 +97,6 @@ File.prototype.transform = function (ast) {
   _.each(transform.transformers, function (transformer) {
     transformer.transform(self);
   });
-
-  return this.generate();
 };
 
 File.prototype.generate = function () {