[CP] [vm/ffi] Support late Finalizable variables

[dcharkes]

Commit(s) to merge

e418026

Target

stable 2.17.1

Issue Description

late variables with (sub)type Finalizable throw LateInitializationErrors when initialized by assignment instead of immediately on the variable declaration.

class Foo implements Finalizable {}

void main() {
  late Foo foo;
  foo = Foo();
  print(foo);
}

Unhandled exception:
LateInitializationError: Local 'foo' has not been initialized.

Finalizables are kept alive by inserting reachabilityFences, and the argument to the fence causes a field-load which get checked for being initailized.

What is the fix

Don't check field-loads for the sentinel value (the value representing that a late field is not initialized) when the field load is an argument to the reachabilityFence method.

Why cherry-pick

We have released NativeFinalizers in Dart 2.17, and any native resource should implement the Finalizable interface.

None of these native resources can then be used in late variables.

(Concrete use case: automatic reference counting for Objective C objects in package:ffigen. cc @liamappelbe @brianquinlan)

Risk

low

Issue link(s)

#49005

Extra Info

cc @mkustermann @mraleph Thanks for your input on the fix!

[athomas]

Merged to stable in edf0ab2 (2.17.1).