Remove one more use of mirrors: Components now have to explicitly sync their fields.

sky/sdk/lib/framework/components2/button_base.dart

Index: sky/sdk/lib/framework/components2/button_base.dart
diff --git a/sky/sdk/lib/framework/components2/button_base.dart b/sky/sdk/lib/framework/components2/button_base.dart
index 8e507f5adc82b03f829cb818abe6d4cedaf83ec1..c8a9c37e62909d516bb4652330169cd13a9ff3e3 100644
--- a/sky/sdk/lib/framework/components2/button_base.dart
+++ b/sky/sdk/lib/framework/components2/button_base.dart
@@ -6,21 +6,14 @@ import '../fn2.dart';
 
 abstract class ButtonBase extends Component {
 
-  ButtonBase({ Object key }) : super(key: key);
+  ButtonBase({ Object key, this.highlight: false }) : super(key: key);
 
-  UINode buildContent();
+  bool highlight;
 
-  UINode build() {
-    return new EventListenerNode(
-      buildContent(),
-      onPointerDown: _handlePointerDown,
-      onPointerUp: _handlePointerUp,
-      onPointerCancel: _handlePointerCancel
-    );
+  void syncFields(ButtonBase source) {
+    highlight = source.highlight;
   }
 
-  bool highlight = false;
-
   void _handlePointerDown(_) {
     setState(() {
       highlight = true;
@@ -36,4 +29,16 @@ abstract class ButtonBase extends Component {
       highlight = false;
     });
   }
+
+  UINode build() {
+    return new EventListenerNode(
+      buildContent(),
+      onPointerDown: _handlePointerDown,
+      onPointerUp: _handlePointerUp,
+      onPointerCancel: _handlePointerCancel
+    );
+  }
+
+  UINode buildContent();
+
 }