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-react-constant-elements/src/index.js

@@ -1,7 +1,7 @@
 export default function () {
-  var immutabilityVisitor = {
+  let immutabilityVisitor = {
     enter(path, state) {
-      var stop = () => {
+      let stop = () => {
         state.isImmutable = false;
         path.stop();
       };
@@ -28,7 +28,7 @@ export default function () {
       JSXElement(path) {
         if (path.node._hoisted) return;
 
-        var state = { isImmutable: true };
+        let state = { isImmutable: true };
         path.traverse(immutabilityVisitor, state);
 
         if (state.isImmutable) {