Fix: Added `margin` parameter for `MaterialBanner` class (#119005)

* Fix: Added Margin Parameter for Material Banner

* Fix: Comment for default value added and test improved

* Fix: Comment updated

* Fix: Comment added

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

@@ -106,6 +106,7 @@ class MaterialBanner extends StatefulWidget {
     this.shadowColor,
     this.dividerColor,
     this.padding,
+    this.margin,
     this.leadingPadding,
     this.forceActionsBelow = false,
     this.overflowAlignment = OverflowBarAlignment.end,
@@ -185,6 +186,12 @@ class MaterialBanner extends StatefulWidget {
   /// `EdgeInsetsDirectional.only(start: 16.0, top: 2.0)`.
   final EdgeInsetsGeometry? padding;
 
+  /// Empty space to surround the [MaterialBanner].
+  ///
+  /// If the [margin] is null then this defaults to
+  /// 0 if the banner's [elevation] is 0, 10 otherwise.
+  final EdgeInsetsGeometry? margin;
+
   /// The amount of space by which to inset the [leading] widget.
   ///
   /// This defaults to `EdgeInsetsDirectional.only(end: 16.0)`.
@@ -237,6 +244,7 @@ class MaterialBanner extends StatefulWidget {
       leading: leading,
       backgroundColor: backgroundColor,
       padding: padding,
+      margin: margin,
       leadingPadding: leadingPadding,
       forceActionsBelow: forceActionsBelow,
       overflowAlignment: overflowAlignment,
@@ -318,6 +326,7 @@ class _MaterialBannerState extends State<MaterialBanner> {
     );
 
     final double elevation = widget.elevation ?? bannerTheme.elevation ?? 0.0;
+    final EdgeInsetsGeometry margin = widget.margin ?? EdgeInsets.only(bottom: elevation > 0 ? 10.0 : 0.0);
     final Color backgroundColor = widget.backgroundColor
         ?? bannerTheme.backgroundColor
         ?? defaults.backgroundColor!;
@@ -334,7 +343,7 @@ class _MaterialBannerState extends State<MaterialBanner> {
         ?? defaults.contentTextStyle;
 
     Widget materialBanner = Container(
-      margin: EdgeInsets.only(bottom: elevation > 0 ? 10.0 : 0.0),
+      margin: margin,
       child: Material(
         elevation: elevation,
         color: backgroundColor,

packages/flutter/test/material/banner_test.dart

@@ -1105,6 +1105,28 @@ void main() {
       ),
     );
   });
+
+   testWidgets('Custom Margin respected', (WidgetTester tester) async {
+    const EdgeInsets margin = EdgeInsets.all(30);
+    await tester.pumpWidget(
+      MaterialApp(
+        home: MaterialBanner(
+         margin: margin,
+          content: const Text('I am a banner'),
+          actions: <Widget>[
+            TextButton(
+              child: const Text('Action'),
+              onPressed: () { },
+            ),
+          ],
+        ),
+      ),
+    );
+
+    final Offset topLeft = tester.getTopLeft(find.descendant(of: find.byType(MaterialBanner), matching: find.byType(Material)).first);
+    /// Compare the offset of banner from top left
+    expect(topLeft.dx, margin.left);
+  });
 }
 
 Material _getMaterialFromBanner(WidgetTester tester) {