add support for optional transformers

lib/6to5/file.js

@@ -10,9 +10,10 @@ var t         = require("./types");
 var _         = require("lodash");
 
 function File(opts) {
-  this.opts = File.normaliseOptions(opts);
-  this.uids = {};
-  this.ast  = {};
+  this.opts         = File.normaliseOptions(opts);
+  this.transformers = this.getTransformers();
+  this.uids         = {};
+  this.ast          = {};
 }
 
 File.declarations = [
@@ -39,6 +40,7 @@ File.normaliseOptions = function (opts) {
     blacklist:    [],
     whitelist:    [],
     sourceMap:    false,
+    optional:     [],
     comments:     true,
     filename:     "unknown",
     modules:      "common",
@@ -80,10 +82,24 @@ File.normaliseOptions = function (opts) {
 
   transform._ensureTransformerNames("blacklist", opts.blacklist);
   transform._ensureTransformerNames("whitelist", opts.whitelist);
+  transform._ensureTransformerNames("optional", opts.optional);
 
   return opts;
 };
 
+File.prototype.getTransformers = function () {
+  var file = this;
+  var transformers = [];
+
+  _.each(transform.transformers, function (transformer) {
+    if (transformer.canRun(file)) {
+      transformers.push(transformer);
+    }
+  });
+
+  return transformers;
+};
+
 File.prototype.toArray = function (node, i) {
   if (t.isArrayExpression(node)) {
     return node;
@@ -182,15 +198,25 @@ File.prototype.parse = function (code) {
 };
 
 File.prototype.transform = function (ast) {
+  var self = this;
+
   this.ast = ast;
   this.scope = new Scope(ast.program);
   this.moduleFormatter = this.getModuleFormatter(this.opts.modules);
 
-  var self = this;
+  var astRun = function (key) {
+    _.each(self.transformers, function (transformer) {
+      transformer.astRun(self, key);
+    });
+  };
 
-  _.each(transform.transformers, function (transformer) {
+  astRun("enter");
+
+  _.each(this.transformers, function (transformer) {
     transformer.transform(self);
   });
+
+  astRun("exit");
 };
 
 File.prototype.generate = function () {