Generate names array for sourcemaps

The Mozilla source-map library already supported the names array, it just wasn't getting passed the right param. I'm not checking the type of the node because only nodes that we care about should have a `name` property. If that's not kosher it would be straightforward to check that `node.type === Syntax.Identifier` first before passing it into `new SourceNode()`, up to you guys!

Tried to follow test format closely. If I didn't get it quite right I'd love feedback!