Include widget configuration in the template (#6476)

This should help new users figure out how to set up their constructors
and use config data from the build method of a State.

packages/flutter_tools/templates/create/lib/main.dart.tmpl

@@ -14,7 +14,6 @@ void main() {
 
 class MyApp extends StatelessWidget {
   // This widget is the root of your application.
-  // It is stateless, meaning that it always looks the same.
   @override
   Widget build(BuildContext context) {
     return new MaterialApp(
@@ -30,18 +29,25 @@ class MyApp extends StatelessWidget {
         // reset back to zero -- the application is not restarted.
         primarySwatch: Colors.blue,
       ),
-      home: new MyHomePage(),
+      home: new MyHomePage(title: 'Flutter Demo'),
     );
   }
 }
 
 class MyHomePage extends StatefulWidget {
-  MyHomePage({Key key}) : super(key: key);
+  MyHomePage({Key key, this.title}) : super(key: key);
 
   // This widget is the home page of your application. It is stateful,
   // meaning that it has a State object (defined below) that contains
   // fields that affect how it looks.
 
+  // This class is the configuration for the state. It holds the
+  // values (in this case the title) provided by the parent (in this
+  // case the App widget) and used by the build method of the State.
+  // Fields in a Widget subclass are always marked "final".
+
+  final String title;
+
   @override
   _MyHomePageState createState() => new _MyHomePageState();
 }
@@ -65,7 +71,10 @@ class _MyHomePageState extends State<MyHomePage> {
   Widget build(BuildContext context) {
     return new Scaffold(
       appBar: new AppBar(
-        title: new Text('Flutter Demo'),
+        // Here we take the value from the MyHomePage object that
+        // was created by the App.build method, and use it to set
+        // our appbar title.
+        title: new Text(config.title),
       ),
       body: new Center(
         child: new Text(