[NNBD] Migrate sample code pt 3 (#72837)

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

@@ -66,7 +66,7 @@ enum BottomNavigationBarType {
 ///    case it's assumed that each item will have a different background color
 ///    and that background color will contrast well with white.
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 /// This example shows a [BottomNavigationBar] as it is used within a [Scaffold]
 /// widget. The [BottomNavigationBar] has three [BottomNavigationBarItem]
 /// widgets and the [currentIndex] is set to index 0. The selected item is

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

@@ -596,7 +596,7 @@ class _BottomSheetSuspendedCurve extends ParametricCurve<double> {
 /// Returns a `Future` that resolves to the value (if any) that was passed to
 /// [Navigator.pop] when the modal bottom sheet was closed.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold}
 ///
 /// This example demonstrates how to use `showModalBottomSheet` to display a
 /// bottom sheet that obscures the content behind it when a user taps a button.

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

@@ -148,7 +148,7 @@ class ExpansionPanelRadio extends ExpansionPanel {
 /// Note that [expansionCallback] behaves differently for [ExpansionPanelList]
 /// and [ExpansionPanelList.radio].
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold}
 ///
 /// Here is a simple example of how to implement ExpansionPanelList.
 ///
@@ -156,8 +156,8 @@ class ExpansionPanelRadio extends ExpansionPanel {
 /// // stores ExpansionPanel state information
 /// class Item {
 ///   Item({
-///     this.expandedValue,
-///     this.headerValue,
+///     required this.expandedValue,
+///     required this.headerValue,
 ///     this.isExpanded = false,
 ///   });
 ///
@@ -252,7 +252,7 @@ class ExpansionPanelList extends StatefulWidget {
   /// arguments must not be null. The [children] objects must be instances
   /// of [ExpansionPanelRadio].
   ///
-  /// {@tool dartpad --template=stateful_widget_scaffold_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_scaffold}
   ///
   /// Here is a simple example of how to implement ExpansionPanelList.radio.
   ///
@@ -260,9 +260,9 @@ class ExpansionPanelList extends StatefulWidget {
   /// // stores ExpansionPanel state information
   /// class Item {
   ///   Item({
-  ///     this.id,
-  ///     this.expandedValue,
-  ///     this.headerValue,
+  ///     required this.id,
+  ///     required this.expandedValue,
+  ///     required this.headerValue,
   ///   });
   ///
   ///   int id;

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

@@ -53,7 +53,7 @@ enum StretchMode {
 /// [FlexibleSpaceBar.createSettings], to convey sizing information down to the
 /// [FlexibleSpaceBar].
 ///
-/// {@tool dartpad --template=freeform_no_null_safety}
+/// {@tool dartpad --template=freeform}
 /// This sample application demonstrates the different features of the
 /// [FlexibleSpaceBar] when used in a [SliverAppBar]. This app bar is configured
 /// to stretch into the overscroll space, and uses the
@@ -78,7 +78,7 @@ enum StretchMode {
 ///             stretch: true,
 ///             onStretchTrigger: () {
 ///               // Function callback for stretch
-///               return;
+///               return Future.value();
 ///             },
 ///             expandedHeight: 300.0,
 ///             flexibleSpace: FlexibleSpaceBar(

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

@@ -431,7 +431,7 @@ abstract class FloatingActionButtonLocation {
 /// You can create your own subclass of [StandardFabLocation]
 /// to implement a custom [FloatingActionButtonLocation].
 ///
-/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_material}
 ///
 /// This is an example of a user-defined [FloatingActionButtonLocation].
 ///

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

@@ -40,7 +40,7 @@ const double _kMinButtonSize = kMinInteractiveDimension;
 /// requirements in the Material Design specification. The [alignment] controls
 /// how the icon itself is positioned within the hit region.
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_scaffold_center}
 ///
 /// This sample shows an `IconButton` that uses the Material icon "volume_up" to
 /// increase the volume.
@@ -84,7 +84,7 @@ const double _kMinButtonSize = kMinInteractiveDimension;
 /// the underlying [Material] along with the splash and highlight
 /// [InkResponse] contributed by descendant widgets.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold}
 ///
 /// In this sample the icon button's background color is defined with an [Ink]
 /// widget whose child is an [IconButton]. The icon button's filled background

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

@@ -57,7 +57,7 @@ const Duration _kElevationDuration = Duration(milliseconds: 75);
 /// Outline buttons have a minimum size of 88.0 by 36.0 which can be overridden
 /// with [ButtonTheme].
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_center_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold_center}
 ///
 /// Here is an example of a basic [OutlineButton].
 ///

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

@@ -243,12 +243,12 @@ class _LinearProgressIndicatorPainter extends CustomPainter {
 /// The minimum height of the indicator can be specified using [minHeight].
 /// The indicator can be made taller by wrapping the widget with a [SizedBox].
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// This example shows a [LinearProgressIndicator] with a changing value.
 ///
 /// ```dart
-///  AnimationController controller;
+///  late AnimationController controller;
 ///
 ///  @override
 ///  void initState() {
@@ -481,12 +481,12 @@ class _CircularProgressIndicatorPainter extends CustomPainter {
 /// The indicator arc is displayed with [valueColor], an animated value. To
 /// specify a constant color use: `AlwaysStoppedAnimation<Color>(color)`.
 ///
-/// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// This example shows a [CircularProgressIndicator] with a changing value.
 ///
 /// ```dart
-///  AnimationController controller;
+///  late AnimationController controller;
 ///
 ///  @override
 ///  void initState() {