Revision 50973 added this test, and some infrastructure to all ports. We may need to implement something similar to what Mac/Win have in DRT, and also perform the same changes they did in FrameLoaderClient.
So, from my quick investigation, isolated worlds looks very much like what we were looking for when we were talking about using seed to do javascript inside WebKit - it is meant to isolate the global contexts, and allow specific contexts to have a bigger privilege. We will need to add API to deal with worlds - JSC doesn't deal with them, they are a WebCore concept, and we will have to extend our window-object-cleared signal to also give a World object (we'll need to have a wrapper here).
Is there still any interest for this API?
We have WebKitScriptWorld in the WebKit2 injected bundle API now.