Migrate sample code to NNBD (#72755)

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

@@ -12,9 +12,8 @@ import 'interface_level.dart';
 import 'theme.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// Widget child;
-// BuildContext context;
+// late Widget child;
+// late BuildContext context;
 
 /// A palette of [Color] constants that describe colors commonly used when
 /// matching the iOS platform aesthetics.

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

@@ -8,9 +8,6 @@ import 'package:flutter/widgets.dart';
 import 'colors.dart';
 import 'theme.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 // Content padding determined via SwiftUI's `Form` view in the iOS 14.2 SDK.
 const EdgeInsetsGeometry _kDefaultPadding =
     EdgeInsetsDirectional.fromSTEB(20.0, 6.0, 6.0, 6.0);
@@ -45,7 +42,7 @@ const EdgeInsetsGeometry _kDefaultPadding =
 ///
 /// ```dart
 /// class FlutterDemo extends StatefulWidget {
-///   FlutterDemo({Key key}) : super(key: key);
+///   FlutterDemo({Key? key}) : super(key: key);
 ///
 ///   @override
 ///   _FlutterDemoState createState() => _FlutterDemoState();

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

@@ -11,9 +11,6 @@ import 'package:flutter/widgets.dart';
 
 import 'theme.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 // Minimum padding from edges of the segmented control to edges of
 // encompassing widget.
 const EdgeInsetsGeometry _kHorizontalItemPadding = EdgeInsets.symmetric(horizontal: 16.0);
@@ -141,7 +138,7 @@ class CupertinoSegmentedControl<T extends Object> extends StatefulWidget {
   ///     1: Text('Child 2'),
   ///   };
   ///
-  ///   int currentValue;
+  ///   late int currentValue;
   ///
   ///   @override
   ///   Widget build(BuildContext context) {

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

@@ -16,9 +16,6 @@ import 'theme.dart';
 
 export 'package:flutter/services.dart' show TextInputType, TextInputAction, TextCapitalization, SmartQuotesType, SmartDashesType;
 
-// Examples can assume:
-// // @dart = 2.9
-
 const TextStyle _kDefaultPlaceholderStyle = TextStyle(
   fontWeight: FontWeight.w400,
   color: CupertinoColors.placeholderText,
@@ -154,7 +151,7 @@ class _CupertinoTextFieldSelectionGestureDetectorBuilder extends TextSelectionGe
 /// }
 ///
 /// class _MyPrefilledTextState extends State<MyPrefilledText> {
-///   TextEditingController _textController;
+///   late TextEditingController _textController;
 ///
 ///   @override
 ///   void initState() {

packages/flutter/lib/src/physics/gravity_simulation.dart

@@ -5,8 +5,7 @@
 import 'simulation.dart';
 
 // Examples can assume:
-// // @dart = 2.9
-// AnimationController _controller;
+// late AnimationController _controller;
 
 /// A simulation that applies a constant accelerating force.
 ///

packages/flutter/lib/src/rendering/binding.dart

@@ -20,7 +20,6 @@ import 'view.dart';
 export 'package:flutter/gestures.dart' show HitTestResult;
 
 // Examples can assume:
-// // @dart = 2.9
 // dynamic context;
 
 /// The glue between the render tree and the Flutter engine.
@@ -221,7 +220,7 @@ mixin RendererBinding on BindingBase, ServicesBinding, SchedulerBinding, Gesture
   /// Querying [PlatformDispatcher.platformBrightness].
   ///
   /// ```dart
-  /// final Brightness brightness = WidgetsBinding.instance.platformDispatcher.platformBrightness;
+  /// final Brightness brightness = WidgetsBinding.instance!.platformDispatcher.platformBrightness;
   /// ```
   /// {@end-tool}
   ///

packages/flutter/lib/src/rendering/box.dart

@@ -13,9 +13,6 @@ import 'package:vector_math/vector_math_64.dart';
 import 'debug.dart';
 import 'object.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 // This class should only be used in debug builds.
 class _DebugSize extends Size {
   _DebugSize(Size source, this._owner, this._canBeUsedByParent) : super.copy(source);
@@ -718,7 +715,7 @@ class BoxHitTestResult extends HitTestResult {
   ///   }
   ///
   ///   @override
-  ///   bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
+  ///   bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
   ///     return result.addWithPaintTransform(
   ///       transform: _effectiveTransform,
   ///       position: position,

packages/flutter/lib/src/rendering/debug_overflow_indicator.dart

@@ -11,9 +11,6 @@ import 'package:flutter/foundation.dart';
 import 'object.dart';
 import 'stack.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 // Describes which side the region data overflows on.
 enum _OverflowSide {
   left,
@@ -57,20 +54,20 @@ class _OverflowRegionData {
 /// ```dart
 /// class MyRenderObject extends RenderAligningShiftedBox with DebugOverflowIndicatorMixin {
 ///   MyRenderObject({
-///     AlignmentGeometry alignment,
-///     TextDirection textDirection,
-///     RenderBox child,
+///     AlignmentGeometry alignment = Alignment.center,
+///     TextDirection? textDirection,
+///     RenderBox? child,
 ///   }) : super.mixin(alignment, textDirection, child);
 ///
-///   Rect _containerRect;
-///   Rect _childRect;
+///   late Rect _containerRect;
+///   late Rect _childRect;
 ///
 ///   @override
 ///   void performLayout() {
 ///     // ...
-///     final BoxParentData childParentData = child.parentData;
+///     final BoxParentData childParentData = child!.parentData! as BoxParentData;
 ///     _containerRect = Offset.zero & size;
-///     _childRect = childParentData.offset & child.size;
+///     _childRect = childParentData.offset & child!.size;
 ///   }
 ///
 ///   @override

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -28,9 +28,6 @@ export 'package:flutter/gestures.dart' show
   PointerUpEvent,
   PointerCancelEvent;
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// A base class for render boxes that resemble their children.
 ///
 /// A proxy box has a single child and simply mimics all the properties of that
@@ -3053,7 +3050,7 @@ class RenderRepaintBoundary extends RenderProxyBox {
   ///
   /// ```dart
   /// class PngHome extends StatefulWidget {
-  ///   PngHome({Key key}) : super(key: key);
+  ///   PngHome({Key? key}) : super(key: key);
   ///
   ///   @override
   ///   _PngHomeState createState() => _PngHomeState();
@@ -3063,10 +3060,10 @@ class RenderRepaintBoundary extends RenderProxyBox {
   ///   GlobalKey globalKey = GlobalKey();
   ///
   ///   Future<void> _capturePng() async {
-  ///     RenderRepaintBoundary boundary = globalKey.currentContext.findRenderObject();
+  ///     RenderRepaintBoundary boundary = globalKey.currentContext!.findRenderObject()! as RenderRepaintBoundary;
   ///     ui.Image image = await boundary.toImage();
-  ///     ByteData byteData = await image.toByteData(format: ui.ImageByteFormat.png);
-  ///     Uint8List pngBytes = byteData.buffer.asUint8List();
+  ///     ByteData? byteData = await image.toByteData(format: ui.ImageByteFormat.png);
+  ///     Uint8List pngBytes = byteData!.buffer.asUint8List();
   ///     print(pngBytes);
   ///   }
   ///