6.0.0

I'm extremely stupid and didn't commit as I go. To anyone reading this
I'm extremely sorry. A lot of these changes are very broad and I plan on
releasing Babel 6.0.0 today live on stage at Ember Camp London so I'm
afraid I couldn't wait. If you're ever in London I'll buy you a beer
(or assorted beverage!) to make up for it, also I'll kiss your feet and
give you a back massage, maybe.

packages/babel-plugin-transform-undefined-to-void/src/index.js

@@ -1,9 +1,9 @@
 export default function ({ types: t }) {
   return {
     visitor: {
-      ReferencedIdentifier(node) {
-        if (node.name === "undefined") {
-          return t.unaryExpression("void", t.numberLiteral(0), true);
+      ReferencedIdentifier(path) {
+        if (path.node.name === "undefined") {
+          path.replaceWith(t.unaryExpression("void", t.numberLiteral(0), true));
         }
       }
     }