Add LayoutBuilder example (#61948)

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

@@ -247,6 +247,59 @@ mixin RenderConstrainedLayoutBuilder<ConstraintType extends Constraints, ChildTy
 /// in an [Align] widget. If the child might want to be bigger, consider
 /// wrapping it in a [SingleChildScrollView] or [OverflowBox].
 ///
+/// {@tool dartpad --template=stateless_widget_material}
+///
+/// This example uses a [LayoutBuilder] to build a different widget depending on the available width. Resize the
+/// DartPad window to see [LayoutBuilder] in action!
+///
+/// ```dart
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     appBar: AppBar(title: Text("LayoutBuilder Example")),
+///     body: LayoutBuilder(
+///       builder: (context, constraints) {
+///         if (constraints.maxWidth > 600) {
+///           return _buildWideContainers();
+///         } else {
+///           return _buildNormalContainer();
+///         }
+///       },
+///     ),
+///   );
+/// }
+///
+/// Widget _buildNormalContainer() {
+///   return Center(
+///     child: Container(
+///       height: 100.0,
+///       width: 100.0,
+///       color: Colors.red,
+///     ),
+///   );
+/// }
+///
+/// Widget _buildWideContainers() {
+///   return Center(
+///     child: Row(
+///       mainAxisAlignment: MainAxisAlignment.spaceEvenly,
+///       children: <Widget>[
+///         Container(
+///           height: 100.0,
+///           width: 100.0,
+///           color: Colors.red,
+///         ),
+///         Container(
+///           height: 100.0,
+///           width: 100.0,
+///           color: Colors.yellow,
+///         ),
+///       ],
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [SliverLayoutBuilder], the sliver counterpart of this widget.