anchor.diff:    QrDemo(CanvasElement canvas, DivElement typeDiv, DivElement errorDiv) :
       _canvas = canvas,
       _ctx = canvas.context2D,
  -    _qrMapper = new _QrCalc(),
  +    _qrMapper = new ThrottledStream(_calc),
       _scale = new BungeeNum(1) {
       _ctx.fillStyle = 'black';
   
  -    _qrMapper.outputChanged.listen((args) {
  -      _squares = _qrMapper.output;
  +    _qrMapper.outputStream.listen((args) {
  +      _squares = _qrMapper.outputValue;
         requestFrame();
       });
   
  
positive.code:   static final String _errorLevelIdKey = 'error-value';
    final BungeeNum _scale;
    final CanvasElement _canvas;
    final _QrCalc _qrMapper;
    final CanvasRenderingContext2D _ctx;
  
    String _value = '';
  
negative.code: import 'dart:html';
  import 'dart:async';
  import 'dart:math' as math;
  import 'package:bot/bot.dart';
  import 'package:bot_web/bot_html.dart';
  import 'package:qr/qr.dart';
  import 'package:qr/src/send_port_value.dart';
  
  void main() {
    final CanvasElement canvas = querySelector("#content");
    final DivElement typeDiv = querySelector('#type-div');
    final DivElement errorDiv = querySelector('#error-div');
  