Migrate some sample code (#72558)

packages/flutter/lib/src/painting/continuous_rectangle_border.dart

@@ -93,7 +93,7 @@ class ContinuousRectangleBorder extends OutlinedBorder {
     final double top = rrect.top;
     final double bottom = rrect.bottom;
     //  Radii will be clamped to the value of the shortest side
-    /// of [rrect] to avoid strange tie-fighter shapes.
+    // of rrect to avoid strange tie-fighter shapes.
     final double tlRadiusX =
       math.max(0.0, _clampToShortest(rrect, rrect.tlRadiusX));
     final double tlRadiusY =

packages/flutter/lib/src/painting/gradient.dart

@@ -330,7 +330,7 @@ abstract class Gradient {
 /// Typically this class is used with [BoxDecoration], which does the painting.
 /// To use a [LinearGradient] to paint on a canvas directly, see [createShader].
 ///
-/// {@tool dartpad --template=stateless_widget_material_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_material}
 /// This sample draws a picture that looks like vertical window shades by having
 /// a [Container] display a [BoxDecoration] with a [LinearGradient].
 ///

packages/flutter/lib/src/painting/image_stream.dart

@@ -9,9 +9,6 @@ import 'dart:ui' show hashValues;
 import 'package:flutter/foundation.dart';
 import 'package:flutter/scheduler.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// A [dart:ui.Image] object with its corresponding scale.
 ///
 /// ImageInfo objects are used by [ImageStream] objects to represent the
@@ -69,8 +66,8 @@ class ImageInfo {
   /// [ImageInfo] reference refers to new image data or not.
   ///
   /// ```dart
-  /// ImageInfo _imageInfo;
-  /// set imageInfo (ImageInfo value) {
+  /// ImageInfo? _imageInfo;
+  /// set imageInfo (ImageInfo? value) {
   ///   // If the image reference is exactly the same, do nothing.
   ///   if (value == _imageInfo) {
   ///     return;
@@ -78,7 +75,7 @@ class ImageInfo {
   ///   // If it is a clone of the current reference, we must dispose of it and
   ///   // can do so immediately. Since the underlying image has not changed,
   ///   // We don't have any additional work to do here.
-  ///   if (value != null && _imageInfo != null && value.isCloneOf(_imageInfo)) {
+  ///   if (value != null && _imageInfo != null && value.isCloneOf(_imageInfo!)) {
   ///     value.dispose();
   ///     return;
   ///   }

packages/flutter/lib/src/painting/text_span.dart

@@ -13,9 +13,6 @@ import 'inline_span.dart';
 import 'text_painter.dart';
 import 'text_style.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-
 /// An immutable span of text.
 ///
 /// A [TextSpan] object can be styled using its [style] property. The style will
@@ -128,7 +125,7 @@ class TextSpan extends InlineSpan {
   /// }
   ///
   /// class _BuzzingTextState extends State<BuzzingText> {
-  ///   LongPressGestureRecognizer _longPressRecognizer;
+  ///   late LongPressGestureRecognizer _longPressRecognizer;
   ///
   ///   @override
   ///   void initState() {

packages/flutter/lib/src/painting/text_style.dart

@@ -25,8 +25,7 @@ const String _kColorBackgroundWarning = 'Cannot provide both a backgroundColor a
 const double _kDefaultFontSize = 14.0;
 
 // Examples can assume:
-// // @dart = 2.9
-// BuildContext context;
+// late BuildContext context;
 
 /// An immutable style describing how to format and paint text.
 ///
@@ -205,7 +204,7 @@ const double _kDefaultFontSize = 14.0;
 ///         foreground: Paint()
 ///           ..style = PaintingStyle.stroke
 ///           ..strokeWidth = 6
-///           ..color = Colors.blue[700],
+///           ..color = Colors.blue[700]!,
 ///       ),
 ///     ),
 ///     // Solid text as fill.