sendTask method
发送任务给子isolate
Implementation
void sendTask(Future Function() task, IsolateMessage isolate,
void Function(dynamic) callback) {
final port = ReceivePort();
isolate.receivePort = port;
// 向Isolate发送任务
isolate.sendPort.send(<dynamic>[task, port.sendPort]);
isolate.msgBackPort = port;
// 监听任务完成的通知
port.listen((message) {
callback(message);
});
}