TreeIterator: Tests and Fixes

**Work In Progress.**

Continuing to improve our tests. This time — `TreeIterator`.

Implementing tests according to `Mozilla AST`. Found one unexpected behaviour.
![6c63e3fb4e22c4dbc69342ccc8cb448a60f04f6fbfa818ebf6a3bb79fc2067a6](https://cloud.githubusercontent.com/assets/945528/5267072/4da33920-7a5f-11e4-8a0a-68e323a3c0af.jpg)