Add RoundedSuperellipseBorder and apply it to CupertinoActionSheet (flutter#166303)

This PR creates a new class `RoundedSuperellipseBorder`, which is the
main way to draw a rounded superellipse with filling and/or stroking.

The new class is very similar to `RoundedRectangleBorder` and shares a
lot of private code, therefore they reside in the same file.

For demonstration purposes, the rounded superellipse is also applied to
`CupertinoActionSheet`, whose cancel button was drawn with the border
class.



https://github.com/user-attachments/assets/39599dcf-5cf1-46e1-ab34-8c477cbef9d4

(Sadly this demo wouldn't fit for dartpad because rounded superellipses
are not yet supported on Web.)

## Pre-launch Checklist

- [ ] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [ ] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [ ] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: dkwingsmt

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

@@ -1299,7 +1299,7 @@ class _CupertinoActionSheetState extends State<CupertinoActionSheet> {
 
     final List<Widget> children = <Widget>[
       Flexible(
-        child: ClipRRect(
+        child: ClipRSuperellipse(
           borderRadius: const BorderRadius.all(Radius.circular(12.0)),
           child: BackdropFilter(
             filter: ImageFilter.blur(
@@ -1593,24 +1593,30 @@ class _ActionSheetButtonBackgroundState extends State<_ActionSheetButtonBackgrou
 
   @override
   Widget build(BuildContext context) {
-    late final Color backgroundColor;
-    BorderRadius? borderRadius;
+    late final Widget child;
     if (!widget.isCancel) {
-      backgroundColor = widget.pressed ? _kActionSheetPressedColor : _kActionSheetBackgroundColor;
+      child = ColoredBox(
+        color: CupertinoDynamicColor.resolve(
+          widget.pressed ? _kActionSheetPressedColor : _kActionSheetBackgroundColor,
+          context,
+        ),
+        child: widget.child,
+      );
     } else {
-      backgroundColor = widget.pressed ? _kActionSheetCancelPressedColor : _kActionSheetCancelColor;
-      borderRadius = const BorderRadius.all(Radius.circular(_kCornerRadius));
-    }
-    return MetaData(
-      metaData: this,
-      child: Container(
-        decoration: BoxDecoration(
-          color: CupertinoDynamicColor.resolve(backgroundColor, context),
-          borderRadius: borderRadius,
+      child = DecoratedBox(
+        decoration: ShapeDecoration(
+          shape: const RoundedSuperellipseBorder(
+            borderRadius: BorderRadius.all(Radius.circular(_kCornerRadius)),
+          ),
+          color: CupertinoDynamicColor.resolve(
+            widget.pressed ? _kActionSheetCancelPressedColor : _kActionSheetCancelColor,
+            context,
+          ),
         ),
         child: widget.child,
-      ),
-    );
+      );
+    }
+    return MetaData(metaData: this, child: child);
   }
 }
 

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

@@ -24,9 +24,11 @@ import 'edge_insets.dart';
 /// interpolated or animated. The [Border] class cannot interpolate between
 /// different shapes.
 enum BoxShape {
-  /// An axis-aligned, 2D rectangle. May have rounded corners (described by a
-  /// [BorderRadius]). The edges of the rectangle will match the edges of the box
-  /// into which the [Border] or [BoxDecoration] is painted.
+  /// An axis-aligned rectangle, optionally with rounded corners.
+  ///
+  /// The amount of corner rounding, if any, is determined by the border radius
+  /// specified by classes such as [BoxDecoration] or [Border]. The rectangle's
+  /// edges match those of the box in which it is painted.
   ///
   /// See also:
   ///

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

@@ -28,8 +28,9 @@ import 'image_provider.dart';
 ///
 /// The box has a [border], a body, and may cast a [boxShadow].
 ///
-/// The [shape] of the box can be a circle or a rectangle. If it is a rectangle,
-/// then the [borderRadius] property controls the roundness of the corners.
+/// The [shape] of the box can be [BoxShape.circle] or [BoxShape.rectangle]. If
+/// it is [BoxShape.rectangle], then the [borderRadius] property can be used to
+/// make it a rounded rectangle ([RRect]).
 ///
 /// The body of the box is painted in layers. The bottom-most layer is the
 /// [color], which fills the box. Above that is the [gradient], which also fills