implemented stateful_widget example

stateful_widget/lib/main.dart

@@ -0,0 +1,56 @@
+import 'package:flutter/material.dart';
+
+void main() {
+  runApp(new MaterialApp(
+    home: new MyButton(),
+  ));
+}
+
+class MyButton extends StatefulWidget {
+  @override
+  MyButtonState createState() {
+    return new MyButtonState();
+  }
+}
+
+class MyButtonState extends State<MyButton> {
+  int counter = 0;
+  List<String> strings = ['Flutter', 'is', 'cool', "and","awesome!"];
+  String displayedString = "Hello World!";
+
+  void onPressOfButton() {
+    setState(() {
+      displayedString = strings[counter];
+      counter = counter < 4 ? counter + 1 : 0;
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new Scaffold(
+      appBar: new AppBar(
+        title: new Text("Stateful Widget"),
+        backgroundColor: Colors.green,
+      ),
+      body: new Container(
+        child: new Center(
+          child: new Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            children: <Widget>[
+              new Text(displayedString, style: new TextStyle(fontSize: 40.0)),
+              new Padding(padding: new EdgeInsets.all(10.0)),
+              new RaisedButton(
+                child: new Text(
+                  "Press me",
+                  style: new TextStyle(color: Colors.white),
+                ),
+                color: Colors.red,
+                onPressed: onPressOfButton,
+              )
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}