How do I do the equivalent of setTimeout + clearTimeout in Dart?

You can use the Timer class

import 'dart:async';

var timer = Timer(Duration(seconds: 1), () => print('done'));

timer.cancel();

If you want to mimic the JavaScript API:

import 'dart:async';

Timer setTimeout(callback, [int duration = 1000]) {
  return Timer(Duration(milliseconds: duration), callback);
}

void clearTimeout(Timer t) {
  t.cancel();
}