Right now we have widgets (e.g. Navigator, Router) that interact with the SystemNavigator API to talk to the engine (especially the web engine). However, this means things break if you ever have multiple routers or navigators or other SystemNavigator clients interacting simultaneously.

As @chunhtai suggested in flutter/engine#26164, we should consider creating a singleton API hanging off the binding that multiplexes clients of the navigation API so that, for example, multiple parallel Router widgets can coexist and function meaningfully in the browser history, just like a web page with multiple iframes.