Add BorderRadiusGeometry to Divider Widget for Customisable Border Radius (#163414)

This PR fixes #162239 . Now, you can add border radius to divider. It is
needed when you have a thick divider and almost all the thick dividers
need a radius.

Example Usage:

```diff
  Divider(
    height: 20,
    thickness: 5,
    color: Colors.blue,
+   radius: BorderRadius.all(Radius.circular(20)), 
  );
```

```diff
  VerticalDivider(
    height: 20,
    thickness: 5,
    color: Colors.blue,
+   radius: BorderRadius.all(Radius.circular(20)), 
  );
```


## Pre-launch Checklist

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

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

Co-authored-by: Tong Mu <dkwingsmt@users.noreply.github.com>

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

@@ -58,11 +58,18 @@ class Divider extends StatelessWidget {
   ///
   /// The [height], [thickness], [indent], and [endIndent] must be null or
   /// non-negative.
-  const Divider({super.key, this.height, this.thickness, this.indent, this.endIndent, this.color})
-    : assert(height == null || height >= 0.0),
-      assert(thickness == null || thickness >= 0.0),
-      assert(indent == null || indent >= 0.0),
-      assert(endIndent == null || endIndent >= 0.0);
+  const Divider({
+    super.key,
+    this.height,
+    this.thickness,
+    this.indent,
+    this.endIndent,
+    this.color,
+    this.radius,
+  }) : assert(height == null || height >= 0.0),
+       assert(thickness == null || thickness >= 0.0),
+       assert(indent == null || indent >= 0.0),
+       assert(endIndent == null || endIndent >= 0.0);
 
   /// The divider's height extent.
   ///
@@ -94,6 +101,11 @@ class Divider extends StatelessWidget {
   /// also null, then this defaults to 0.0.
   final double? endIndent;
 
+  /// The amount of radius for the border of the divider.
+  ///
+  /// If this is null, then the default radius of [BoxDecoration] will be used.
+  final BorderRadiusGeometry? radius;
+
   /// The color to use when painting the line.
   ///
   /// If this is null, then the [DividerThemeData.color] is used. If that is
@@ -177,6 +189,7 @@ class Divider extends StatelessWidget {
           height: thickness,
           margin: EdgeInsetsDirectional.only(start: indent, end: endIndent),
           decoration: BoxDecoration(
+            borderRadius: radius,
             border: Border(bottom: createBorderSide(context, color: color, width: thickness)),
           ),
         ),
@@ -227,6 +240,7 @@ class VerticalDivider extends StatelessWidget {
     this.indent,
     this.endIndent,
     this.color,
+    this.radius,
   }) : assert(width == null || width >= 0.0),
        assert(thickness == null || thickness >= 0.0),
        assert(indent == null || indent >= 0.0),
@@ -277,6 +291,11 @@ class VerticalDivider extends StatelessWidget {
   /// {@end-tool}
   final Color? color;
 
+  /// The amount of radius for the border of the divider.
+  ///
+  /// If this is null, then the default radius of [BoxDecoration] will be used.
+  final BorderRadiusGeometry? radius;
+
   @override
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
@@ -295,6 +314,7 @@ class VerticalDivider extends StatelessWidget {
           width: thickness,
           margin: EdgeInsetsDirectional.only(top: indent, bottom: endIndent),
           decoration: BoxDecoration(
+            borderRadius: radius,
             border: Border(left: Divider.createBorderSide(context, color: color, width: thickness)),
           ),
         ),

packages/flutter/test/material/divider_test.dart

@@ -40,6 +40,22 @@ void main() {
     expect(decoration.border!.bottom.width, 5.0);
   });
 
+  testWidgets('Divider custom radius', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(child: Divider(radius: BorderRadius.circular(5))),
+      ),
+    );
+    final Container container = tester.widget(find.byType(Container));
+    final BoxDecoration decoration = container.decoration! as BoxDecoration;
+    final BorderRadius borderRadius = decoration.borderRadius! as BorderRadius;
+    expect(borderRadius.bottomLeft, const Radius.circular(5));
+    expect(borderRadius.bottomRight, const Radius.circular(5));
+    expect(borderRadius.topLeft, const Radius.circular(5));
+    expect(borderRadius.topRight, const Radius.circular(5));
+  });
+
   testWidgets('Horizontal divider custom indentation', (WidgetTester tester) async {
     const double customIndent = 10.0;
     Rect dividerRect;
@@ -183,6 +199,22 @@ void main() {
     expect(lineRect.bottom, dividerRect.bottom - customIndent);
   });
 
+  testWidgets('VerticalDivider custom radius', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(child: VerticalDivider(radius: BorderRadius.circular(5))),
+      ),
+    );
+    final Container container = tester.widget(find.byType(Container));
+    final BoxDecoration decoration = container.decoration! as BoxDecoration;
+    final BorderRadius borderRadius = decoration.borderRadius! as BorderRadius;
+    expect(borderRadius.bottomLeft, const Radius.circular(5));
+    expect(borderRadius.bottomRight, const Radius.circular(5));
+    expect(borderRadius.topLeft, const Radius.circular(5));
+    expect(borderRadius.topRight, const Radius.circular(5));
+  });
+
   // Regression test for https://github.com/flutter/flutter/issues/39533
   testWidgets('createBorderSide does not throw exception with null context', (
     WidgetTester tester,