Arbitrary closures can be stored into vm:shared fields

[alexmarkov]

This opens up the possibility to share arbitrary objects.

Example:

import 'package:dart_internal/isolate_group.dart' show IsolateGroup;
import 'dart:isolate';

@pragma('vm:shared')
List<String> Function()? getList;

void main(List<String> args) {
  final list = <String>[];
  getList = () => list;
  void func() {
    final list = getList!();
    for (int i = 0; i < 1000; i++) {
      list.add('abc$i');
      list.removeLast();
    }
  };
  for (int i = 0; i < 20; ++i) {
    Isolate.run(() {
      IsolateGroup.runSync(func);
    });
  }
}

dart --experimental-shared-data foo.dart

===== CRASH =====
si_signo=Segmentation fault(11), si_code=SEGV_ACCERR(2), si_addr=0x7fe1c378adf0
version=3.11.0-edge (main) (Unknown timestamp) on "linux_x64"
pid=1594953, thread=1594966, isolate_group=main(0x557d32857970), isolate=(nil)((nil))
os=linux, arch=x64, comp=no, sim=no

dart --experimental-shared-data foo.dart

Unhandled exception:
type 'Null' is not a subtype of type 'String' in type cast
#0      List.length= (dart:core-patch/growable_array.dart:236:12)
#1      List.removeLast (dart:core-patch/growable_array.dart:346:10)
#2      main.func

dart --experimental-shared-data foo.dart

Unhandled exception:
RangeError (index): Invalid value: Not in inclusive range 0..19: 21
#0      List._setIndexed (dart:core-patch/growable_array.dart:281:49)
#1      List.[]= (dart:core-patch/growable_array.dart:276:5)
#2      List.add (dart:core-patch/growable_array.dart:291:9)
#3      main.func

/cc @aam