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

sky/examples/stocks2/lib/stock_arrow.dart

Index: sky/examples/stocks2/lib/stock_arrow.dart
diff --git a/sky/examples/stocks2/lib/stock_arrow.dart b/sky/examples/stocks2/lib/stock_arrow.dart
index a13cbe41998f85a7a469a334fc2c0fd2e41295cc..cadfb7159ca80e47cdeb23e7a0c213b793a6bc64 100644
--- a/sky/examples/stocks2/lib/stock_arrow.dart
+++ b/sky/examples/stocks2/lib/stock_arrow.dart
@@ -16,6 +16,11 @@ class StockArrow extends Component {
 
   StockArrow({ Object key, this.percentChange }) : super(key: key);
 
+  void syncFields(StockArrow source) {
+    percentChange = source.percentChange;
+    super.syncFields(source);
+  }
+
   int _colorIndexForPercentChange(double percentChange) {
     double maxPercent = 10.0;
     double normalizedPercentChange = math.min(percentChange.abs(), maxPercent) / maxPercent;