title: Flutter Navigation push Replacement is not working when not placed in the first activity
body: <p>I have several Page. <br>
  1. Start Page <br>
  2. Page 2<br>
  3. Page 3<br>
  4. Main Menu<br></p>
  
  <p>From 1 -> 2. and 2 -> 3. i use this for navigation : </p>
  
  <pre><code>Navigator.of(context).push(new MaterialPageRoute&lt;MyPage&gt;(
                      builder: (BuildContext context) {
                        return new MyPage();
                      },
                    ));
  </code></pre>
  
  <p>and for 3 -> 4. I want to use this (<strong>Push Replacement</strong>, will not going back), but it doesnt work and act like normal Push:</p>
  
  <pre><code>Navigator
          .of(context)
          .pushReplacement(new MaterialPageRoute(builder: (BuildContext context) {
        return new MainMenuPage();
      }));
  </code></pre>
  
  <p>Confusing.</p>