`ProcessSignal.sigint.watch()` does not work correctly with `dart.exe`

[bkonyi]

This was discovered when we noticed that the Flutter tool was leaking processes on Windows and realized that the shutdown hooks were not executing correctly when the process was sent SIGINT (see flutter/flutter#173616).

To reproduce, run the following program using dart.exe and dartvm.exe:

// signal.dart
import 'dart:io';

main() async {
  ProcessSignal.sigint.watch().listen((_) {
     print('SIGINT RECEIVED');
     exit(0);
  });
  
  while(true) {
    await Future.delayed(const Duration(seconds: 1));
    print('Waiting...');
  }
}

When run from source using dartvm.exe, sending SIGINT results in the signal handler being invoked:

PS C:\dart_sdk\sdk> C:\Users\tenpi\flutter\bin\cache\dart-sdk\bin\dartvm.exe signal.dart
Waiting...
Waiting...
SIGINT RECEIVED
PS C:\dart_sdk\sdk>

However, when running from source using dart.exe, sending SIGINT results in the process being killed, but the signal handler isn't invoked (at least, it fails to run most of the time):

PS C:\dart_sdk\sdk> C:\Users\tenpi\flutter\bin\cache\dart-sdk\bin\dart.exe signal.dart  
Waiting...
Waiting...
PS C:\dart_sdk\sdk> 

This is almost certainly a regression from the recent migration of dart.exe to exec(...) dartvm.exe when running Dart code from source.