End off the day with Poller.

- Renamed util.js to internal.js. util.js has actual utilities exposed to the library's API now.
- util.js currently has Poller (it really had no place in node, so I placed it here). It also has a small implementation of Dart's Duration, since there is nothing like it in JavaScript without an external dependency. It's nothing to get too excited about, but it's extremely useful for working with Pollers.
- Made a file for trends, I'll actually work on them tomorrow.
- Small test change, just copied from the Dart SDK repository.