Added browser compatibility tests

This is in the spirit of https://github.com/dart-lang/html5lib/issues/24

I've added a new set of tests that can be run in the console via `html5lib/dom.dart` and via browser and `dart:html`

The goal is to ensure ongoing compatibility between the subset of API that are important to both.

The two tests I've created are simple, but already hit some non-obvious issues.