I think too that Isolates are the best approach but wasn't successful using them on the browser a while ago, but there was a bigger refactoring going on lately in Isolates.
Does anyone know of a current client side Isolates example? The API doc referenced by @kevmoo contains a link to an Isolates article that doesn't exist anymore (maybe must be rewritten due to the mentioned refactoring).
Another approach would be a method that returns after a chunk of work and gets recalled repeated in a loop until it returns for example true for done (false for not yet).
When you call this method using scheduleMicrotask(doChunk)
or new Timer(() => doChunk())
other tasks get some air (import 'dart:async';
) each time before the method gets actually called.