问使用Dart2js长度和[]时的ElementList警告

EN

import 'dart:math';
import 'dart:html';
import 'dart:async';

const String CLICK_ME = 'Click me to pause!';

void randomColoring(StreamSubscription subscription) {

  final Random random = new Random();
  final Element intro = querySelector("#intro");
  final ElementList<Element> boxes = querySelectorAll('.box');
  final Element info = querySelector("#info");

  subscription.onData((eventValue) {
    var box = random.nextInt(boxes.length);                      /// WARNING
    var hexCssColor = random.nextInt(0xFFFFFF + 1).toRadixString(16)
        .padLeft(6, "0").toUpperCase();
    boxes[box].style.background = "#$hexCssColor";               /// WARNING
    info.text = eventValue.toString();
    print("box: $box - hexCssColor: $hexCssColor - count: $eventValue");
  });

  var text = intro.text;
  intro
      ..text = CLICK_ME
      ..onClick.listen((mouseEvent) {
        if (subscription.isPaused) {
          intro.text = CLICK_ME;
          subscription.resume();
        } else {
          intro.text = text;
          subscription.pause();
        }
      });
}