Teach Sky buttons and dialogs how to use the new Theme system

sky/sdk/lib/widgets/material.dart

Index: sky/sdk/lib/widgets/material.dart
diff --git a/sky/sdk/lib/widgets/material.dart b/sky/sdk/lib/widgets/material.dart
index 73b52cb66f8372c9efd909ff6fa42a766af5cf7b..864cf1a6af204ac7dd1dba5c20582b17a2492a37 100644
--- a/sky/sdk/lib/widgets/material.dart
+++ b/sky/sdk/lib/widgets/material.dart
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import '../painting/box_painter.dart';
+import '../theme/colors.dart' as colors;
 import '../theme/edges.dart';
 import '../theme/shadows.dart';
 import 'basic.dart';
@@ -24,6 +25,17 @@ class Material extends Component {
   final MaterialEdge edge;
   final Color color;
 
+  Color get backgroundColor {
+    if (color != null)
+      return color;
+    switch (Theme.of(this).brightness) {
+      case ThemeBrightness.light:
+        return colors.Grey[50];
+      case ThemeBrightness.dark:
+        return colors.Grey[850];
+    }
+  }
+
   // TODO(ianh): we should make this animate level changes and color changes
 
   Widget build() {
@@ -31,7 +43,7 @@ class Material extends Component {
       decoration: new BoxDecoration(
         boxShadow: shadows[level],
         borderRadius: edges[edge],
-        backgroundColor: color == null ? Theme.of(this).backgroundColor : color,
+        backgroundColor: backgroundColor,
         shape: edge == MaterialEdge.circle ? Shape.circle : Shape.rectangle
       ),
       child: new DefaultTextStyle(style: Theme.of(this).text.body1, child: child)