* babel-traverse: add test to demonstrate repeated identifiers being evaluated to `false`
* fix typo: `value` -> `val`
* path.evaluate: only mark item as resolved if we're confident
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.