Extension test harness

The start of a test harness for chrome extensions. Not totally sure this is the best way to test extensions but its a start. Load the unpacked extension then create a new tab and load a web page. Click the dart icon on the top right of the browser. 

```
chrome.browserAction.onClicked.addListener(function(tab) {
  chrome.tabs.create({url: "harness_extension.html"}, function() {
  	console.log("launching harness_extension.html");
  });
});
```
Once loaded just rebuild the javascript code and refresh the `harness_extension.html` page. @devoncarew have you been able to run in the unit tests from the dartvm? I forgot what tricks might be needed for this.  
@DrMarcII

![screen shot 2013-07-10 at 8 45 59 pm](https://f.cloud.github.com/assets/654526/779570/55e2fd72-e9de-11e2-9e09-45784a8ad711.png)