I really dig the flexibility of transport. Having something that works over postMessage is totally clutch!!

> Similarly, supports passing objects by reference: If a class extends the special marker type RpcTarget, then instances of that class are passed by reference, with method calls calling back to the location where the object was created.

Can this be relaxed? Having to design the object model ahead of time for RpcTarget is constraining. If we could just attach a ThingClass.prototype[Symbol.for('RpcTarget')] = true then there would be a lot more flexibility, less need to design explciitly for RpcTarget, to use RpcTarget with the objects/classes of 3rd party libraries.

The fear here is that if a class wasn't explicitly designed to be an RPC interface, then it could very easily offer functionality that isn't safe to expose over a security boundary with RPC. Normally, JavaScript classes do not expect their APIs to be security boundaries.

With that said, I do think we ought to support `new RpcStub(myObject)` to explicitly create a stub around an arbitrary class, even if it doesn't extend `RpcTarget`. It would be up to the person writing the `new RpcStub` invocation to verify it's safe.