I took the code from the Chrome Demo Extension (http://developer.chrome.com/extensions/getstarted.html) and replaced pretty much everything with dart.
I also kept the dart dummy code.
In Dartium I went to Settings > Extensions > Developer mode > Load unpacked extension
(choose the 'web' folder) and load the code to see if it works. It does ;-)
To see if you can build a Chrome extension in Dart, test it in Dartium, and then use dart2js to generate the javascript required for a 'normal' Chrome extension.