Migrated some cupertino doc comments to null safety. (#72766)

packages/flutter/lib/src/cupertino/app.dart

@@ -13,9 +13,6 @@ import 'localizations.dart';
 import 'route.dart';
 import 'theme.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// An application that uses Cupertino design.
 ///
 /// A convenience widget that wraps a number of widgets that are commonly
@@ -277,7 +274,7 @@ class CupertinoApp extends StatefulWidget {
   ///       LogicalKeySet(LogicalKeyboardKey.select): const ActivateIntent(),
   ///     },
   ///     color: const Color(0xFFFF0000),
-  ///     builder: (BuildContext context, Widget child) {
+  ///     builder: (BuildContext context, Widget? child) {
   ///       return const Placeholder();
   ///     },
   ///   );
@@ -310,7 +307,7 @@ class CupertinoApp extends StatefulWidget {
   ///       ),
   ///     },
   ///     color: const Color(0xFFFF0000),
-  ///     builder: (BuildContext context, Widget child) {
+  ///     builder: (BuildContext context, Widget? child) {
   ///       return const Placeholder();
   ///     },
   ///   );

packages/flutter/lib/src/cupertino/context_menu.dart

@@ -75,7 +75,7 @@ enum _ContextMenuLocation {
 /// child's corners and allowing its aspect ratio to expand, similar to the
 /// Photos app on iOS.
 ///
-/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_material}
 ///
 /// This sample shows a very simple CupertinoContextMenu for an empty red
 /// 100x100 Container. Simply long press on it to open.

packages/flutter/lib/src/cupertino/search_field.dart

@@ -12,9 +12,6 @@ import 'icons.dart';
 import 'localizations.dart';
 import 'text_field.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// A [CupertinoTextField] that mimics the look and behavior of UIKit's
 /// `UISearchTextField`.
 ///
@@ -35,7 +32,7 @@ import 'text_field.dart';
 /// }
 ///
 /// class _MyPrefilledSearchState extends State<MyPrefilledSearch> {
-///   TextEditingController _textController;
+///   late TextEditingController _textController;
 ///
 ///   @override
 ///   void initState() {

packages/flutter/lib/src/cupertino/slider.dart

@@ -14,7 +14,6 @@ import 'theme.dart';
 import 'thumb_painter.dart';
 
 // Examples can assume:
-// // @dart = 2.9
 // int _cupertinoSliderValue = 1;
 // void setState(VoidCallback fn) { }
 

packages/flutter/lib/src/cupertino/sliding_segmented_control.dart

@@ -12,9 +12,6 @@ import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 // Extracted from https://developer.apple.com/design/resources/.
 
 // Minimum padding from edges of the segmented control to edges of
@@ -197,14 +194,14 @@ class CupertinoSlidingSegmentedControl<T> extends StatefulWidget {
   ///     1: Text('Child 2'),
   ///   };
   ///
-  ///   int currentValue;
+  ///   int? currentValue;
   ///
   ///   @override
   ///   Widget build(BuildContext context) {
   ///     return Container(
   ///       child: CupertinoSlidingSegmentedControl<int>(
   ///         children: children,
-  ///         onValueChanged: (int newValue) {
+  ///         onValueChanged: (int? newValue) {
   ///           setState(() {
   ///             currentValue = newValue;
   ///           });

packages/flutter/lib/src/cupertino/switch.dart

@@ -14,8 +14,7 @@ import 'colors.dart';
 import 'thumb_painter.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// bool _lights;
+// bool _lights = false;
 // void setState(VoidCallback fn) { }
 
 /// An iOS-style switch.

packages/flutter/lib/src/cupertino/text_form_field_row.dart

@@ -11,9 +11,6 @@ import 'form_row.dart';
 import 'text_field.dart';
 import 'theme.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// Creates a [CupertinoFormRow] containing a [FormField] that wraps
 /// a [CupertinoTextField].
 ///
@@ -64,18 +61,18 @@ import 'theme.dart';
 /// ```dart
 /// CupertinoTextFormFieldRow(
 ///   prefix: Text('Username'),
-///   onSaved: (String value) {
+///   onSaved: (String? value) {
 ///     // This optional block of code can be used to run
 ///     // code when the user saves the form.
 ///   },
-///   validator: (String value) {
-///     return value.contains('@') ? 'Do not use the @ char.' : null;
+///   validator: (String? value) {
+///     return (value != null && value.contains('@')) ? 'Do not use the @ char.' : null;
 ///   },
 /// )
 /// ```
 /// {@end-tool}
 ///
-/// {@tool dartpad --template=stateful_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateful_widget_material}
 /// This example shows how to move the focus to the next field when the user
 /// presses the SPACE key.
 ///
@@ -90,7 +87,7 @@ import 'theme.dart';
 ///       child: Form(
 ///         autovalidateMode: AutovalidateMode.always,
 ///         onChanged: () {
-///           Form.of(primaryFocus.context).save();
+///           Form.of(primaryFocus!.context!)?.save();
 ///         },
 ///         child: CupertinoFormSection.insetGrouped(
 ///           header: Text('SECTION 1'),
@@ -99,7 +96,7 @@ import 'theme.dart';
 ///               prefix: Text('Enter text'),
 ///               placeholder: 'Enter text',
 ///               validator: (value) {
-///                 if (value.isEmpty) {
+///                 if (value == null || value.isEmpty) {
 ///                   return 'Please enter a value';
 ///                 }
 ///                 return null;