Allow sending a generic object with a function type parameter between isolates

[dnfield]

See #37294

This code fails, now with a meaningful error message:

import 'dart:isolate';

void fun() {
 print('hi from fun');
}

Future<void> main() async {
  final x = <void Function()>[fun];
  await Isolate.spawn(isolate, x);
}

void isolate(List<void Function()> foo) async {
  print('Tick: $foo');
  foo.first();
}

But this code works:

import 'dart:isolate';

void fun() {
  print('hi from fun');
}

Future<void> main() async {
  final x = <dynamic>[fun];
  await Isolate.spawn(isolate, x);
}

void isolate(List<dynamic> foo) async {
  print('Tick: $foo');
  (foo.first as void Function())();
}

It'd be nice to let the typed version work too.

[dnfield]

(it also works with a List<Object>)

[mkustermann]

After we're done with #36097 this would probably implicitly work because we we'll share function type objects across isolates.

[mkustermann]

Closing this issue, merge into the more general #46623.