Add shapeBorder option on App Bar (#21834)

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

@@ -150,6 +150,7 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
     this.flexibleSpace,
     this.bottom,
     this.elevation,
+    this.shape,
     this.backgroundColor,
     this.brightness,
     this.iconTheme,
@@ -286,6 +287,12 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   /// for app bars.
   final double elevation;
 
+  /// The material's shape as well its shadow.
+  ///
+  /// A shadow is only displayed if the [elevation] is greater than
+  /// zero.
+  final ShapeBorder shape;
+
   /// The color to use for the app bar's material. Typically this should be set
   /// along with [brightness], [iconTheme], [textTheme].
   ///
@@ -583,6 +590,7 @@ class _AppBarState extends State<AppBar> {
           elevation: widget.elevation
             ?? appBarTheme.elevation
             ?? _defaultElevation,
+          shape: widget.shape,
           child: Semantics(
             explicitChildNodes: true,
             child: appBar,

packages/flutter/test/material/app_bar_test.dart

@@ -1485,4 +1485,33 @@ void main() {
     await gesture.up();
     await tester.pump();
   });
+
+  testWidgets('AppBar with shape', (WidgetTester tester) async {
+    const RoundedRectangleBorder roundedRectangleBorder = RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(15.0)));
+    await tester.pumpWidget(
+      MaterialApp(
+        home: AppBar(
+          leading: const Text('L'),
+          title: const Text('No Scaffold'),
+          shape: roundedRectangleBorder,
+          actions: const <Widget>[Text('A1'), Text('A2')],
+        ),
+      ),
+    );
+
+    final Finder appBarFinder = find.byType(AppBar);
+
+    AppBar getAppBarWidget() {
+      return tester.widget<AppBar>(appBarFinder);
+    }
+
+    expect(getAppBarWidget().shape, roundedRectangleBorder);
+
+    final Finder materialFinder = find.byType(Material);
+    Material getMaterialWidget() {
+      return tester.widget<Material>(materialFinder);
+    }
+
+    expect(getMaterialWidget().shape, roundedRectangleBorder);
+  });
 }