title: Update SnackBar content in Flutter
body: <p>I have a button that displays a <code>SnackBar</code> (or toast) before moving to the next page. I have a countdown and after 5 seconds I push Page2.</p>
  
  <pre><code>RaisedButton(
                      onPressed: () {
                        _startTimer();
                        final snackBar = SnackBar(
                          behavior: SnackBarBehavior.floating,
                          content: Text(
                              'Prepare yourself to start in ${widget._current.toString()}!'), // doesn't work here
                          duration: new Duration(seconds: widget._start),
                          action: SnackBarAction(
                            label: widget._current.toString(), // and neither does here
                            onPressed: () {
                              // Some code to undo the change.
                            },
                          ),
                        );
                        Scaffold.of(context).showSnackBar(snackBar);
                      },
                      child: Text(
                        "I'm ready",
                        style: TextStyle(fontSize: 20),
                      ),
                    ),
  </code></pre>
  
  <p>Nothing to see on the countdown but I'll paste it just in case:</p>
  
  <pre><code>void _startTimer() {
      CountdownTimer countDownTimer = new CountdownTimer(
        new Duration(seconds: widget._start),
        new Duration(seconds: 1),
      );
  
      var sub = countDownTimer.listen(null);
      sub.onData((duration) {
        setState(() {
          widget._current = widget._start - duration.elapsed.inSeconds;
        });
      });
  
      sub.onDone(() {
        print("Done");
        sub.cancel();
      });
    }
  </code></pre>
  
  <p>So if I display the countdown somewhere else (inside a <code>Text</code> for example) it works but it seems that the <code>SnackBar</code> doesn't change its contain, it always get the max number of the countdown.</p>