I jumped the gun a bit with the documentation, its in the CMIS 1.1 spec Section 5 Browser binding, its pretty good, with a bit of interpretation its OK. Another reason I was having probs was that my proxy setup was wrong, this was confusing things.
So we now have a reasonably functional browser based CMIS client written in Dart. It only allows repository/root folder/type info, document and folder CRUD and queries but its all I need for now.
so it is actually possible to write something for the browser binding. Thanks for sharing your code. Sadly I'm not into the Dart-language, so I will ask two questions directly.