Updated the Dart benchmarks for M4 release

Dart has come a long way in the last year. This should bring us instep with with Dart's M4 release and current best-practices.

- Dependencies are now managed by Pub
- Code is split into `bench2d.dart` (benchmarking from command-line) and `web/Bench2dWeb.dart` (debugging in browser)
- Browser version now uses standard `dart.js` to handle JS fallback

Cheers!