Although Dart is moving full speed forward becoming (hopefully) a swiss army knife, there is a field which is both challenging but a must for a new language to be embraced from JS developers too (and not mostly from GWT and Java devs).
Talking about putting foundation bricks for in-house ui framework, using/combining LESS or SASS with CSS for simple components/widgets like buttons, containers, panels, lists, datagrid development, etc. etc.
So anything about building a gui framework can do :)
Most of the book is currently built around building an MVC framework, so there is a fair bit about this already in there. It's a lightweight framework similar to Backbone.js rather than a heavier one with explicit data bindings. There is a chapter on DOM and, more pertinent, events.
Still, I don't know that I have this as a point of emphasis anywhere, so thanks for pointing it out. I'm going to leave this open as I work through the beta to encourage myself to find ways of talking about this more.