Dart port of Firebase push ID generator (origin: https://gist.github.com/mikelehen/3596a30bd69384624c11)

generate_pushid.dart

import 'dart:math' as math;

//See https://firebase.googleblog.com/2015/02/the-2120-ways-to-ensure-unique_68.html
//https://gist.github.com/amirh/0e7b480691f7e6506317a7600398124b
//Modified to remove hyphen and underscore to make ids url freindly
const String _kPushChars =
    '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
const _pushCharsLength = _kPushChars.length;

class _PushIdGenerator {
  int lastPushTime = 0;
  final List<int> randomSuffix = new List<int>(12);
  final math.Random random = new math.Random.secure();

  String next() {
    final int now = new DateTime.now().toUtc().millisecondsSinceEpoch;
    String id = _toPushIdBase64(now, 8);

    if (now != lastPushTime) {
      for (int i = 0; i < 12; i += 1) {
        randomSuffix[i] = random.nextInt(_pushCharsLength);
      }
    } else {
      int i;
      for (i = 11; i >= 0 && randomSuffix[i] == _pushCharsLength - 1; i--)
        randomSuffix[i] = 0;
      randomSuffix[i] += 1;
    }
    final String suffixStr = randomSuffix.map((int i) => _kPushChars[i]).join();
    lastPushTime = now;

    return '$id$suffixStr';
  }

  String _toPushIdBase64(int value, int numChars) {
    List<String> chars = new List<String>(numChars);
    for (int i = numChars - 1; i >= 0; i -= 1) {
      chars[i] = _kPushChars[value % _pushCharsLength];
      value = (value / _pushCharsLength).floor();
    }
    assert(value == 0);
    return chars.join();
  }
}

_PushIdGenerator _idGenerator = new _PushIdGenerator();

String generateId() => _idGenerator.next();