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.

doc/design/compiler-assumptions.md

@@ -0,0 +1,9 @@
+# Compiler assumptions
+
+Babel and it's associated official transforms make some assumptions about your code. These
+assumptions are only made as they're either **impossible** to take into consideration or
+are extremely exotic.
+
+ - `undefined`, `NaN` and `Infinity` have not been externally redefined.
+ - Built-in objects such as `Object`, `Array`, `String`, `Number`, `Boolean` etc have not been redefined.
+ - Standard methods on built-ins have not been redefined in a way that breaks the original contract.