This is the site for the Dart Cookbook project.
The Dart Cookbook is being authored by Shailen Tuli.
A Beta version of this book is online:
http://www.dartlang.org/docs/cookbook/
Recipes are generally created in branches (strings, testing, json, etc.) and are merged into master from time to time.
And yes, there are tests. Take a look at the test/ folder and you'll find tests for the code that shows up in the recipes. Again, look in the individual branches as the contents there are more likely to be up to date.
For Web UI recipes, check out the webui or webui_template branches. The sample app for each recipe can be found inside the web/webui directory.
The Dart Cookbook is scheduled for publication by O'Reilly.