Update Card example and shadowColor default (#61392)

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

@@ -7,11 +7,11 @@
 import 'package:flutter/widgets.dart';
 
 import 'card_theme.dart';
-import 'colors.dart';
 import 'material.dart';
 import 'theme.dart';
 
-/// A material design card. A card has slightly rounded corners and a shadow.
+/// A material design card: a panel with slightly rounded corners and an
+/// elevation shadow.
 ///
 /// A card is a sheet of [Material] used to represent some related information,
 /// for example an album, a geographical location, a meal, contact details, etc.
@@ -39,16 +39,19 @@ import 'theme.dart';
 ///             title: Text('The Enchanted Nightingale'),
 ///             subtitle: Text('Music by Julie Gable. Lyrics by Sidney Stein.'),
 ///           ),
-///           ButtonBar(
+///           Row(
+///             mainAxisAlignment: MainAxisAlignment.end,
 ///             children: <Widget>[
-///               FlatButton(
+///               TextButton(
 ///                 child: const Text('BUY TICKETS'),
 ///                 onPressed: () { /* ... */ },
 ///               ),
-///               FlatButton(
+///               const SizedBox(width: 8),
+///               TextButton(
 ///                 child: const Text('LISTEN'),
 ///                 onPressed: () { /* ... */ },
 ///               ),
+///               const SizedBox(width: 8),
 ///             ],
 ///           ),
 ///         ],
@@ -93,7 +96,6 @@ import 'theme.dart';
 /// See also:
 ///
 ///  * [ListTile], to display icons and text in a card.
-///  * [ButtonBar], to display buttons at the bottom of a card.
 ///  * [showDialog], to display a modal card.
 ///  * <https://material.io/design/components/cards.html>
 class Card extends StatelessWidget {
@@ -127,7 +129,8 @@ class Card extends StatelessWidget {
   /// The color to paint the shadow below the card.
   ///
   /// If null then the ambient [CardTheme]'s shadowColor is used.
-  /// If that's null too, then the default is fully opaque black.
+  /// If that's null too, then the overall theme's [ThemeData.shadowColor]
+  /// (default black) is used.
   final Color shadowColor;
 
   /// The z-coordinate at which to place this card. This controls the size of
@@ -191,6 +194,7 @@ class Card extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final ThemeData theme = Theme.of(context);
     final CardTheme cardTheme = CardTheme.of(context);
 
     return Semantics(
@@ -199,8 +203,8 @@ class Card extends StatelessWidget {
         margin: margin ?? cardTheme.margin ?? const EdgeInsets.all(4.0),
         child: Material(
           type: MaterialType.card,
-          shadowColor: shadowColor ?? cardTheme.shadowColor ?? Colors.black,
-          color: color ?? cardTheme.color ?? Theme.of(context).cardColor,
+          shadowColor: shadowColor ?? cardTheme.shadowColor ?? theme.shadowColor,
+          color: color ?? cardTheme.color ?? theme.cardColor,
           elevation: elevation ?? cardTheme.elevation ?? _defaultElevation,
           shape: shape ?? cardTheme.shape ?? const RoundedRectangleBorder(
             borderRadius: BorderRadius.all(Radius.circular(4.0)),

packages/flutter/test/material/card_theme_test.dart

@@ -57,6 +57,7 @@ void main() {
   testWidgets('Card widget properties take priority over theme', (WidgetTester tester) async {
     const Clip clip = Clip.hardEdge;
     const Color color = Colors.orange;
+    const Color shadowColor = Colors.pink;
     const double elevation = 7.0;
     const EdgeInsets margin = EdgeInsets.all(3.0);
     const ShapeBorder shape = RoundedRectangleBorder(
@@ -69,6 +70,7 @@ void main() {
         body: Card(
           clipBehavior: clip,
           color: color,
+          shadowColor: shadowColor,
           elevation: elevation,
           margin: margin,
           shape: shape,
@@ -81,6 +83,7 @@ void main() {
 
     expect(material.clipBehavior, clip);
     expect(material.color, color);
+    expect(material.shadowColor, shadowColor);
     expect(material.elevation, elevation);
     expect(container.margin, margin);
     expect(material.shape, shape);