I want to write a concurrent web app (using either raw Workers or isolates as workers). The Dart API for isolates is `spawnUri`, which takes a URI that points to a script.

In Dartium, that script is a `.dart` file. In Chrome/Firefox/etc that script is a `.dart.js` file.

I need to detect what runtime I'm in so I can configure the URI for `spawnUri` correctly. What is the best way to do this?