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

generate_pushid.dart

import 'dart:math' as math;

const String _kPushChars = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz';
typedef String StringCallback();

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

  return () {
    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(64);
      }
    }
    else {
      int i;
      for(i = 11; i >= 0 && randomSuffix[i] == 63; 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 % 64];
    value = (value / 64).floor();
  }
  assert(value == 0);
  return chars.join();
}

FYI, there's an OutOfBounds bug on line 24:
https://gist.github.com/mikelehen/3596a30bd69384624c11?permalink_comment_id=4652463#gistcomment-4652463