Dart token bucket rate limiter

import 'dart:async';

class TokenBucket {
  TokenBucket(this.capacity, this.perSecond)
      : _tokens = capacity.toDouble(),
        _last = DateTime.now();
  final int capacity;
  final double perSecond;
  double _tokens;
  DateTime _last;

  bool tryAcquire([int cost = 1]) {
    final now = DateTime.now();
    final elapsed = now.difference(_last).inMicroseconds / 1e6;
    final refilled = _tokens + elapsed * perSecond;
    _tokens = refilled < capacity ? refilled : capacity.toDouble();
    _last = now;
    if (_tokens < cost) return false;
    _tokens -= cost;
    return true;
  }
}