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-helper-get-function-arity/src/index.js

@@ -0,0 +1,14 @@
+/* @flow */
+
+import * as t from "babel-types";
+
+export default function (node): number {
+  let params: Array<Object> = node.params;
+  for (let i = 0; i < params.length; i++) {
+    let param = params[i];
+    if (t.isAssignmentPattern(param) || t.isRestElement(param)) {
+      return i;
+    }
+  }
+  return params.length;
+}