title: setStat() doesn't update the screen
body: <p>I am new in flutter and I have a problem which is where to put the setStat() method. 
  here is a simple code that I made to train, it is a stop watch. in debugging, every thing goes well, but the screen doesn't update, it only shows the initial state which is "00:00:00".</p>
  
  <p>here is the code:</p>
  
  <pre><code>import 'package:flutter/material.dart';
  
  void main() {
    runApp(MyApp());
  }
  
  class MyApp extends StatefulWidget {
    @override
    _MyAppState createState() =&gt; _MyAppState();
  }
  
  var time = new Stopwatch();
  
  class _MyAppState extends State&lt;MyApp&gt; {
    int counter = 0;
    bool stat = false;
    String timeFormate = "00:00:00";
  
    setStat() {
      if (stat) {
        timeFormate = time.elapsed.toString();
      }
    }
  
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        title: 'Timer app pro',
        debugShowCheckedModeBanner: false,
        home: Scaffold(
          appBar: AppBar(
            title: Text("Training fucken app"),
          ),
          body: Container(
            child: Column(children: [
              Text(timeFormate),
              RaisedButton(
                child: Text("Start"),
                onPressed: () {
                  time.start();
                  stat = true;
                  print(time.isRunning);
                  print(time.elapsed.toString());
                },
              ),
              RaisedButton(
                child: Text("Stop"),
                onPressed: () {
                  time.stop();
                  print(time.isRunning);
                  print(time.elapsed.toString());
                },
              ),
              RaisedButton(
                child: Text("Reset"),
                onPressed: () =&gt; time.reset(),
              )
            ]),
          ),
        ),
      );
    }
  }
  
  </code></pre>