Sample code for Icon class (#27968)

* Added sample code for a simple Icon, ref: #21136

* Removed duplicate line.

* Fixed incoreect parameter.

* Update icon.dart

Removed empty prose and added better context for sample code. Changed sample code for better visibility in execution of default app themes.

* Alas, Trailing space.

Nixed trailing space in comments.

packages/flutter/lib/src/widgets/icon.dart

@@ -21,6 +21,20 @@ import 'icon_theme_data.dart';
 /// Typically this is introduced automatically by the [WidgetsApp] or
 /// [MaterialApp].
 ///
+/// {@tool sample}
+///
+/// This example shows how to use [Icon] to create an addition icon, in the
+/// color pink, and 30 pixels in size.
+///
+/// ```dart
+/// Icon(
+///   Icons.add,
+///   color: Colors.pink,
+///   size: 30.0,
+/// )
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [IconButton], for interactive icons.