Add Sample code to SlideTransition (#41009)

dev/snippets/config/templates/README.md

@@ -101,6 +101,11 @@ follows:
   `stateful_widget_scaffold`, except that it wraps the stateful widget with a
   `Scaffold` _and_ a `Center`.
 
+- [`stateful_widget_scaffold_center_freeform_state`](stateful_widget_scaffold_center_freeform_state.tmpl) :
+  Similar to `stateful_widget_scaffold_center` except that the code block has
+  to contain the entire state class defined as:
+  `class _MyStatefulWidgetState extends State<MyStatefulWidget>`
+
 - [`stateless_widget_scaffold`](stateless_widget_scaffold.tmpl) : Similar to
   `stateless_widget_material`, except that it wraps the stateless widget with a
   `Scaffold`.

dev/snippets/config/templates/stateful_widget_scaffold_center_freeform_state.tmpl

+// Flutter code sample for {{id}}
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This Widget is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: Scaffold(
+        appBar: AppBar(title: const Text(_title)),
+        body: Center(
+          child: MyStatefulWidget(),
+        ),
+      ),
+    );
+  }
+}
+
+{{code-preamble}}
+
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+{{code}}

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

@@ -194,6 +194,51 @@ class _AnimatedState extends State<AnimatedWidget> {
 /// animated by a [CurvedAnimation] set to [Curves.elasticIn]:
 /// {@animation 300 378 https://flutter.github.io/assets-for-api-docs/assets/widgets/slide_transition.mp4}
 ///
+/// {@tool snippet --template=stateful_widget_scaffold_center_freeform_state}
+/// The following code implements the [SlideTransition] as seen in the video
+/// above:
+///
+/// ```dart
+/// class _MyStatefulWidgetState extends State<MyStatefulWidget> with SingleTickerProviderStateMixin {
+///   AnimationController _controller;
+///   Animation<Offset> _offsetAnimation;
+///
+///   @override
+///   void initState() {
+///     super.initState();
+///     _controller = AnimationController(
+///       duration: const Duration(seconds: 2),
+///       vsync: this,
+///     )..repeat(reverse: true);
+///     _offsetAnimation = Tween<Offset>(
+///       begin: Offset.zero,
+///       end: const Offset(1.5, 0.0),
+///     ).animate(CurvedAnimation(
+///       parent: _controller,
+///       curve: Curves.elasticIn,
+///     ));
+///   }
+///
+///   @override
+///   void dispose() {
+///     super.dispose();
+///     _controller.dispose();
+///   }
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return SlideTransition(
+///       position: _offsetAnimation,
+///       child: const Padding(
+///         padding: EdgeInsets.all(8.0),
+///         child: FlutterLogo(size: 150.0),
+///       ),
+///     );
+///   }
+/// }
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [AlignTransition], an animated version of an [Align] that animates its