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

sky/sdk/lib/widgets/material_button.dart

Index: sky/sdk/lib/widgets/material_button.dart
diff --git a/sky/sdk/lib/widgets/material_button.dart b/sky/sdk/lib/widgets/material_button.dart
index 965d34d5520e824113d880d37543eb5aac5fba61..f1513e5fbf5543623abe435180848ba3d66a92f0 100644
--- a/sky/sdk/lib/widgets/material_button.dart
+++ b/sky/sdk/lib/widgets/material_button.dart
@@ -7,8 +7,6 @@ import 'button_base.dart';
 import 'ink_well.dart';
 import 'material.dart';
 
-enum MaterialButtonTheme { light, dark }
-
 // Rather than using this class directly, please use FlatButton or RaisedButton.
 abstract class MaterialButton extends ButtonBase {
 
@@ -16,20 +14,17 @@ abstract class MaterialButton extends ButtonBase {
     String key,
     this.child,
     this.enabled: true,
-    this.onPressed,
-    this.theme: MaterialButtonTheme.light
+    this.onPressed
   }) : super(key: key);
 
   Widget child;
   bool enabled;
   Function onPressed;
-  MaterialButtonTheme theme;
 
   void syncFields(MaterialButton source) {
     child = source.child;
     enabled = source.enabled;
     onPressed = source.onPressed;
-    theme = source.theme;
     super.syncFields(source);
   }