> With its combination of static and runtime checks
Do you personally use it? That "strong mode" is a theory-only thing, it doesn't work with any real code, only with code you wrote without real third-party libraries.
You are, at best, confusing Dart 2 with Dart 1; strong mode was one of several Dart 1 modes, but, yes, it's possible to run into problems because dependencies weren't designed for it.
There is no option in Dart 2; everything is the equivalent of strong mode, which eliminates ecosystem problems.
> only with code you wrote without real third-party libraries.
Your claim was true several years ago, but since then the entire ecosystem has migrated onto the new strict type system. It was a monumental amount of work, but, thanks to lots of effort from our users, we're there.
Aside from a small number of more-or-less dead packages, you should be able to use any third-party Dart packages you want while still using the new type system.
There is no option in Dart 2; everything is the equivalent of strong mode, which eliminates ecosystem problems.