Optimize node type lookup

lib/6to5/generation/node/index.js

@@ -13,7 +13,7 @@ var find = function (obj, node, parent) {
   for (var i = 0; i < types.length; i++) {
     var type = types[i];
 
-    if (t["is" + type](node)) {
+    if (t.is(type, node)) {
       var fn = obj[type];
       result = fn(node, parent);
       if (result != null) break;