edit pass on T3,4,5, updated images

src/site/docs/tutorials/polymer-intro/examples/stopwatch/web/tute_stopwatch.dart

-import 'dart:html';
-import 'dart:async';
-import 'package:polymer/polymer.dart';
-
-@CustomTag('tute-stopwatch')
-class TuteStopwatch extends PolymerElement with ObservableMixin {
-  @observable String counter='00:00';
-  
-  Stopwatch mywatch = new Stopwatch();
-  Timer mytimer;
-  
-  ButtonElement stopButton;
-  ButtonElement startButton;
-  ButtonElement resetButton;
-  
-  void inserted() {
-    super.inserted();
-    startButton = getShadowRoot('tute-stopwatch').query('#startButton');
-    stopButton = getShadowRoot('tute-stopwatch').query('#stopButton');
-    resetButton = getShadowRoot('tute-stopwatch').query('#resetButton');
-        
-    stopButton.disabled = true;
-    resetButton.disabled = true;
-  }
-  
-  void removed() {
-    super.removed();
-    mytimer.cancel();
-  }
-  
-  void start(Event e, var detail, Node target) {
-    mywatch.start();
-    var oneSecond = new Duration(seconds:1);
-    mytimer = new Timer.periodic(oneSecond, updateTime);
-    startButton.disabled = true;
-    stopButton.disabled = false;
-    resetButton.disabled = true;
-  }
-  
-  void stop(Event e, var detail, Node target) {
-    mywatch.stop();
-    mytimer.cancel();
-    startButton.disabled = false;
-    resetButton.disabled = false;
-    stopButton.disabled = true;
-  }
-  
-  void reset(Event e, var detail, Node target) {
-    mywatch.reset();
-    counter = '00:00';
-    resetButton.disabled = true;
-  }
-  
-  void updateTime(Timer _) {
-    var s = mywatch.elapsedMilliseconds~/1000;
-    var m = 0;
-    
-    if (s >= 60) { m = s ~/ 60; s = s % 60; }
-      
-    String minute = (m <= 9) ? '0$m' : '$m';
-    String second = (s <= 9) ? '0$s' : '$s';
-    counter = '$minute:$second';
-  }
-}