Falling back to spawnFunction() works as expected in the browser but it turns out that isolates are not allowed to access / change the DOM. This reduces isolates to dumb worker functions. Plug-ins which make contributions to the UI are not possible using this approach.
I created some test scripts for the browser and the VM to test the possibilities you have with isolates. You can find them at GitHub: https://github.com/hpehl/dart-isolates. Let me know what you think.