>My doubts arise from the fact that the standard library for a language makes some opinionated design choices, and in my opinion it's impossible to make an idiomatic library that works the same (and is usable!) in a GC context (existing deployment) and a non-GC context (embedded).

What microcontrollers you have in mind? He mentioned Arduino in the post. It's not like we need Dart for extremely low spec-ed mCs, and we could run GC just fine in higher level mCs for ages...