This fixes an issue that comments were added as leading comments
to expressions, which are not immediately folowing the comment.
This was also reported and fixed in espree, and is basically a port to
babylon. eslint/espree#256
this also fixes an issue in babel-eslint with certain rules babel/babel-eslint#289
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.
