Support for background images on cards, style demo home

sky/sdk/lib/widgets/card.dart

Index: sky/sdk/lib/widgets/card.dart
diff --git a/sky/sdk/lib/widgets/card.dart b/sky/sdk/lib/widgets/card.dart
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+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import '../theme/colors.dart' as colors;
+import '../theme/edges.dart';
+import 'basic.dart';
+import 'material.dart';
+import "theme.dart";
+
+class Card extends Component {
+  Card({ String key, this.child, this.color }) : super(key: key);
+
+  final Widget child;
+  final Color color;
+
+  Color get materialColor {
+    if (color != null)
+      return color;
+    switch (Theme.of(this).brightness) {
+      case ThemeBrightness.light:
+        return colors.White;
+      case ThemeBrightness.dark:
+        return colors.Grey[800];
+    }
+  }
+
+  Widget build() {
+    return new Container(
+      margin: const EdgeDims(8.0, 8.0, 0.0, 8.0),
+      child: new Material(
+        color: materialColor,
+        edge: MaterialEdge.card,
+        level: 2,
+        child: new ClipRRect(
+          xRadius: edges[MaterialEdge.card],
+          yRadius: edges[MaterialEdge.card],
+          child: child
+        )
+      )
+    );
+  }
+}