@jason-simmons Can you please have a look at this draft PR? No need to approve it. I just want to make sure we are on a happy path. This PR follows the work outlined in the Isolate Platform Channel design doc and works for iOS Dart->Host messages (notice there is a framework PR if you want to peek at that too). It probably needs a bit more documentation in the source code. I'm still working on a testing plan, your feedback will help inform that. I'm going to have an integration test in the flutter repo for sure.

Something you might find interesting is the threadsafe_unique_ptr. I came up with that with you in mind since I wanted weak_ptr but you've had some valid concerns about shared_ptr being hard to verify so I think this addresses those in this case where we want a clear single owner.

My plan for this is to get early feedback from you and if everything goes well I'll use this as the template for implementing Dart->Host isolate platform channels across the platforms. I don't have plans on implementing Host->Dart isolate platform channels anytime soon (since it didn't have a clear resolution in the design doc and isn't what most people are asking for).

I'm concerned that allowing access to the root isolate's PlatformConfiguration from multiple threads and isolates will be too confusing and error-prone. Likewise for allowing access to UI thread related Shell services from threads besides the UI thread.

I'm not sure what would be the safest way to implement this. The first potential option that comes to mind would be populating the child isolate's UIDartState::Context with the platform task runner plus a service that can be invoked on the platform thread to dispatch a platform message.
The service would need to hold a weak reference to the PlatformView and wrap PlatformView::HandlePlatformMessage. The service could be passed from the parent isolate to the child isolate via the DartIsolateGroupData.

Would be interested in getting consensus among the engine team about how best to do this (@chinmaygarde @dnfield)

Thanks everyone, I'll get to work cleaning up and putting and the dependent PRs into review today.