added ability to configure shadow in banner (#155296)

Added ability to configure shadow in banner

Issues:  https://github.com/flutter/flutter/issues/154505

Before: 
![before](https://github.com/user-attachments/assets/d8b8e826-bab4-462a-a7ac-191e1597c3c6)

After:
![after](https://github.com/user-attachments/assets/3c4b57e7-182a-4749-af72-b82fdaf25013)

packages/flutter/lib/src/widgets/banner.dart

@@ -17,8 +17,12 @@ import 'framework.dart';
 
 const double _kOffset = 40.0; // distance to bottom of banner, at a 45 degree angle inwards
 const double _kHeight = 12.0; // height of banner
-const double _kBottomOffset = _kOffset + 0.707 * _kHeight; // offset plus sqrt(2)/2 * banner height
+const double _kBottomOffset = _kOffset + math.sqrt1_2 * _kHeight;
 const Rect _kRect = Rect.fromLTWH(-_kOffset, _kOffset - _kHeight, _kOffset * 2.0, _kHeight);
+const BoxShadow _kShadow = BoxShadow(
+    color: Color(0x7F000000),
+    blurRadius: 6.0,
+  );
 
 const Color _kColor = Color(0xA0B71C1C);
 const TextStyle _kTextStyle = TextStyle(
@@ -68,6 +72,7 @@ class BannerPainter extends CustomPainter {
     required this.layoutDirection,
     this.color = _kColor,
     this.textStyle = _kTextStyle,
+    this.shadow = _kShadow,
   }) : super(repaint: PaintingBinding.instance.systemFonts) {
     // TODO(polina-c): stop duplicating code across disposables
     // https://github.com/flutter/flutter/issues/137435
@@ -120,10 +125,12 @@ class BannerPainter extends CustomPainter {
   /// Defaults to bold, white text.
   final TextStyle textStyle;
 
-  static const BoxShadow _shadow = BoxShadow(
+  /// The shadow properties for the banner.
-    color: Color(0x7F000000),
+  ///
-    blurRadius: 6.0,
+  /// Use a [BoxShadow] object to define the shadow's color, blur radius,
-  );
+  /// and spread radius. These properties can be used to create different
+  /// shadow effects.
+  final BoxShadow shadow;
 
   bool _prepared = false;
   TextPainter? _textPainter;
@@ -144,7 +151,7 @@ class BannerPainter extends CustomPainter {
   }
 
   void _prepare() {
-    _paintShadow = _shadow.toPaint();
+    _paintShadow = shadow.toPaint();
     _paintBanner = Paint()
       ..color = color;
     _textPainter?.dispose();
@@ -232,6 +239,7 @@ class Banner extends StatefulWidget {
     this.layoutDirection,
     this.color = _kColor,
     this.textStyle = _kTextStyle,
+    this.shadow = _kShadow,
   });
 
   /// The widget to show behind the banner.
@@ -278,6 +286,13 @@ class Banner extends StatefulWidget {
   /// The style of the text shown on the banner.
   final TextStyle textStyle;
 
+  /// The shadow properties for the banner.
+  ///
+  /// Use a [BoxShadow] object to define the shadow's color, blur radius,
+  /// and spread radius. These properties can be used to create different
+  /// shadow effects.
+  final BoxShadow shadow;
+
   @override
   State<Banner> createState() => _BannerState();
 }
@@ -303,6 +318,7 @@ class _BannerState extends State<Banner> {
       layoutDirection: widget.layoutDirection ?? Directionality.of(context),
       color: widget.color,
       textStyle: widget.textStyle,
+      shadow: widget.shadow,
     );
 
     return CustomPaint(

packages/flutter/test/widgets/banner_test.dart

@@ -295,4 +295,31 @@ void main() {
       areCreateAndDispose,
     );
   });
+
+  testWidgets('Can configure shadow for Banner widget', (WidgetTester tester) async {
+    debugDisableShadows = false;
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: Banner(
+          message: 'Shadow banner',
+          location: BannerLocation.topEnd,
+          shadow: BoxShadow(
+            color: Color(0xFF008000),
+            blurRadius: 8.0,
+          ),
+        ),
+      ),
+    );
+    final Finder customPaint = find.byType(CustomPaint);
+
+    expect(customPaint, findsOneWidget);
+
+    final CustomPaint paintWidget = tester.widget(customPaint);
+    final BannerPainter painter = paintWidget.foregroundPainter! as BannerPainter;
+
+    expect(painter.shadow.color, const Color(0xFF008000));
+    expect(painter.shadow.blurRadius, 8.0);
+    debugDisableShadows = true;
+  });
 }