title: Cannot focus on TextField in new page after navigating in Flutter
body: <p>For some reason, I cannot focus on a TextField after on the next page after navigating. The keyboard would automatically dismiss when the TextField is selected. If I set <code>autofocus: true</code> on the TextField, then the keyboard will infinitely popup and immediately dismiss over and over again. </p>
  
  <p>I encountered this when my app was a reasonable size but I was able to recreate this in a minimal example app.</p>
  
  <p>I am using Dart 2.0.0-dev.55.0 with Flutter beta v0.3.2.</p>
  
  <p>Code for main page:</p>
  
  <pre><code>import 'package:flutter/material.dart';
  import 'settings.dart';
  
  void main() =&gt; runApp(new MyApp());
  
  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return new MaterialApp(
        title: 'Flutter Demo',
        theme: new ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: new MyHomePage(title: 'Flutter Demo Home Page'),
      );
    }
  }
  
  class MyHomePage extends StatefulWidget {
    MyHomePage({Key key, this.title}) : super(key: key);
    final String title;
    @override
    _MyHomePageState createState() =&gt; new _MyHomePageState();
  }
  
  class _MyHomePageState extends State&lt;MyHomePage&gt; {
    @override
    Widget build(BuildContext context) {
      return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: &lt;Widget&gt;[
              Text('Helpful text.',),
              // ===== Where navigation happens =====
              RaisedButton(
                onPressed: () {
                  Navigator.push(context, PageRouteBuilder(
                    pageBuilder: (_, __, ___) =&gt; SettingsPage()));
                },
                child: Text('Go to input page'),
              ),
            ],
          ),
        ),
      );
    }
  }
  </code></pre>
  
  <p>Here is the code for the page with the TextField.</p>
  
  <pre><code>import 'package:flutter/material.dart';
  
  class SettingsPage extends StatefulWidget {
    SettingsPage({Key key}) :
     super(key: key);
  
    @override
    _SettingsPage createState() =&gt; new _SettingsPage();
  }
  
  
  class _SettingsPage extends State&lt;SettingsPage&gt; {
  
    @override
    Widget build(BuildContext context) {
      final key = GlobalKey&lt;ScaffoldState&gt;();
      return Scaffold(
        key: key,
        backgroundColor: Colors.white,
        appBar: AppBar(
          title: Text("Settings"),
        ),
        body: ListView(
          children: &lt;Widget&gt;[
            Text("Enter something"),
            // Can't focus on this widget
            TextField(),
          ],
        ),
      );
    }
  }
  </code></pre>
  
  <p>I can focus fine on TextField on the main page if I put one there, but can't on the one in the Settings page. I assume it has something to do with the keyboard not taking priority over the popped up page? Or what is going on? How I can get the app to just focus on input fields on a navigated page?</p>