isProcessAlive function
Returns true if a process with pid is currently running.
Dart has no portable "is PID alive" API, so this shells out to a
platform-specific tool: kill -0 on POSIX (macOS/Linux) and tasklist
on Windows. Never throws — any failure to invoke the tool is treated as
"not alive".
Implementation
bool isProcessAlive(int pid) {
return Platform.isWindows ? _isProcessAliveWindows(pid) : _isProcessAlivePosix(pid);
}