Extension test harness

Discussion about this PR can be found at https://github.com/dart-gde/chrome.dart/pull/54

![screen shot 2013-07-11 at 5 06 00 pm](https://f.cloud.github.com/assets/654526/785606/e90a62b8-ea86-11e2-9f4c-de9e3c83df37.png)