Allow sending synchronous generators via ports(support copying of SuspendState)

[aam]

Currently it's impossible to send generator-created stream via ports due to vms inability to serialize SuspendState, but it seems reasonable to expect for it to work, copy sender's state over effectively forking the stream. Receiver will pick up the stream where it was forked.

import 'dart:isolate';

Stream<int> evenNumbersGenerator() async* {
  int i = 2;
  while (true) {
    yield i;
    i += 2;
  }
}

main() async {
  final rp = ReceivePort();
  rp.sendPort.send(evenNumbersGenerator());
  rp.close();
}

fails with

Unhandled exception:
Invalid argument(s): Illegal argument in isolate message: (object is a SuspendState)
 <- Context num_variables: 1
 <- Closure: (Object?) => void (from dart:core)
 <- Instance of '_ControllerStream<int>' (from dart:async)

#0      _SendPort._sendInternal (dart:isolate-patch/isolate_patch.dart:249:43)
#1      _SendPort.send (dart:isolate-patch/isolate_patch.dart:230:5)
#2      main (file:///Users/aam/p/d1/sdk/runtime/tests/vm/dart/send_async_generator.dart:14:15)
#3      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:297:19)
#4      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:189:12)

cc @alexmarkov @rmacnak-google

[alexmarkov]

Copying Stream objects when sending them between isolates would not work: listener in another isolate would not be notified when events are generated in the original isolate. It is basically the same reason why sending Future objects to another isolate by copying them doesn't make much sense (the reasoning in #51594 (comment) applies to streams as well).

It is not hard to support sending/copying SuspendState objects within isolate group as we share code between isolates in the same group. We can just clone SuspendState object - see SuspendState::Clone. We can use copying of SuspendState objects to implement sending Iterable objects returned by sync* functions, although it may not be very useful for users as sync* functions are extremely rare.

[aam]

Sure, changed asyncronous to synchronous.

Repro of problem sending synchronous generators:

import 'dart:isolate';

Iterable<int> evenNumbersGenerator() sync* {
  int i = 2;
  while (true) {
    yield i;
    i += 2;
  }
}

main() async {
  final rp = ReceivePort();
  rp.sendPort.send(evenNumbersGenerator());
  rp.close();
}

Unhandled exception:
Invalid argument(s): Illegal argument in isolate message: (object is a SuspendState)
 <- Instance of '_SyncStarIterable<int>' (from dart:async)

#0      _SendPort._sendInternal (dart:isolate-patch/isolate_patch.dart:249:43)
#1      _SendPort.send (dart:isolate-patch/isolate_patch.dart:230:5)
#2      main (file:///Users/aam/p/d1/sdk/runtime/tests/vm/dart/send_async_generator.dart:13:15)
#3      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:297:19)
#4      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:189:12)