improve 6to5-node and fix register hook

lib/6to5/register.js

@@ -1,16 +1,23 @@
 require("./polyfill");
 
 var maps = {};
+var node = require("./node");
 var old  = require.extensions[".js"];
 
+// escodegen lazy requires source-map so we force it to load before we override
+// the built-in requires
+var escodegen = require("escodegen");
+escodegen.generate({ type: "Program", body: [] }, { sourceMap: true });
+
 // TODO: source maps - Error.prepareStackTrace override
 
-require.extensions[".js"] = function (m, filename) {
+require.extensions[".js"] =
+require.extensions[".es6"] = function (m, filename) {
   if (filename.indexOf("node_modules") >= 0) {
     return old.apply(this, arguments);
   }
 
-  var result = exports.transformFileSync(filename, {
+  var result = node.transformFileSync(filename, {
     sourceMapObject: true
   });