☂️ Explore shared memory multithreading

[mraleph]

This is an umbrella issue for exploring possible relaxation of Dart's isolate model and introducing some variation of shared memory multithreading into Dart.

Related:

• It is necessary to implement threads language#333
• Proposal

[mraleph]

One open question about shared everything multithreading is around the cost of barriers required to maintain memory model and type safety. Consider the following code:

class X {
  List v;
  X() : v = [];
}

@pragma('vm:shared')
X x = X();

thread1() {
  x = X();
}

thread2() {
  x.v.length;
}

We don't want read of x.v to return null (or worse - garbage), because writes of x and x.v are reordered by hardware with weak memory model.

We should experiment with adding barrier to measure the synchronization cost. We can start by adding a barrier to GenerateAllocateObjectHelper and doing a Golem run.