added: dropdown_button example

dropdown_button/lib/main.dart

@@ -0,0 +1,19 @@
+import 'package:flutter/material.dart';
+
+void main() => runApp(new MyApp());
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return new MaterialApp(
+      home: new Scaffold(
+        appBar: new AppBar(
+          title: new Text("DropDown Button Example"),
+        ),
+        body: new Container(
+          child: new Center(child: new Text("Hello World")),
+        ),
+      ),
+    );
+  }
+}