> (at least I haven't seen a GC system yet that doesn't suffer from STW GC pauses to some degree).

Well I can already think of three languages that have solved this problem: Erlang, Elixir, ponylang. Every actor has it's own isolated heap with it's own GC. If the actor doesn't allocate then it will never have to stop for garbage collection and even if it does, the pause times are extremely low because the heaps are tiny.

It's surprising to me that modern languages like dart aren't even attempting to pick up this low hanging fruit.

Modern languages are all converging to the same good feature set but don't have anything to differentiate from each other. Kotlin, Groovy, Ceylon, Typescript, Dart? All basically look the same to me, there is no real reason to choose one over the other.