Isolates became to ~100x faster in Dart 2.15

Isolates spawned via the Isolate.spawn() API are now grouped, operate on the same managed heap and can therefore share various VM-internal data structures.

This leads to ~100x faster isolate startup latency, ~10-100x lower per-isolate base memory overhead and ~8x faster inter-isolate communication.

Dart 2.15 is now available in the flutter beta channel, more details can be found here: https://github.com/dart-lang/sdk/blob/main/CHANGELOG.md