void sleep(Duration duration)

Sleep for the duration specified in duration.

Use this with care, as no asynchronous operations can be processed in a isolate while it is blocked in a sleep call.

Source 

void sleep(Duration duration) {
  int milliseconds = duration.inMilliseconds;
  if (milliseconds < 0) {
    throw new ArgumentError("sleep: duration cannot be negative");
  }
  _ProcessUtils._sleep(milliseconds);
}