add file argument to transformer.ast and fix ast transformer resolution bug

lib/6to5/transformer.js

@@ -13,6 +13,7 @@ Transformer.normalise = function (transformer) {
     transformer = { ast: transformer };
   } else {
     _.each(transformer, function (fns, type) {
+      if (type === "ast") return;
       if (_.isFunction(fns)) fns = { enter: fns };
       transformer[type] = fns;
     });
@@ -27,7 +28,7 @@ Transformer.prototype.transform = function (file) {
   var ast         = file.ast;
 
   if (transformer.ast) {
-    transformer.ast(ast);
+    transformer.ast(ast, file);
   }
 
   var build = function (exit) {
@@ -59,6 +60,8 @@ Transformer.prototype.transform = function (file) {
 };
 
 Transformer.prototype.canRun = function (file) {
+  if (this.key[0] === "_") return true;
+
   var opts = file.opts;
 
   var blacklist = opts.blacklist;