Add Sample code for FlatButton #21136 (#27751)

* Update flat_button.dart

* Added smaple code for Flat Button

* Update flat_button.dart

* Revert "Update flat_button.dart"

This reverts commit c8f00dff39e18b267295890bd0c5a86979b58c24.

* Revert "Added smaple code for Flat Button"

This reverts commit adc4234bc7956293a5b56f39701c6967fe3e919a.

* Update flat_button.dart

* Update flat_button.dart

* Update flat_button.dart

* remove white spaces

* remove white spaces

* add two different samples

* Update flat_button.dart

packages/flutter/lib/src/material/flat_button.dart

@@ -40,6 +40,48 @@ import 'theme_data.dart';
 ///
 /// The [clipBehavior] argument must not be null.
 ///
+/// {@tool sample}
+///
+/// This example shows a simple [FlatButton].
+///
+/// ```dart
+/// FlatButton(
+///   onPressed: () {
+///     /*...*/
+///   },
+///   child: Text(
+///     "Flat Button",
+///   ),
+/// )
+/// ```
+/// {@end-tool}
+///
+/// {@tool sample}
+///
+/// This example shows a [FlatButton] that is normally white-on-blue,
+/// with splashes rendered in a different shade of blue.
+/// It turns black-on-grey when disabled.
+/// The button has 8px of padding on each side, and the text is 20px high.
+///
+/// ```dart
+/// FlatButton(
+///   color: Colors.blue,
+///   textColor: Colors.white,
+///   disabledColor: Colors.grey,
+///   disabledTextColor: Colors.black,
+///   padding: EdgeInsets.all(8.0),
+///   splashColor: Colors.blueAccent,
+///   onPressed: () {
+///     /*...*/
+///   },
+///   child: Text(
+///     "Flat Button",
+///     style: TextStyle(fontSize: 20.0),
+///   ),
+/// )
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [RaisedButton], a filled button whose material elevates when pressed.