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..3f2bab6ff2fbc7b1bc2b5edfdedf5600e67336c0 100644
--- a/sky/examples/stocks2/lib/stock_arrow.dart
+++ b/sky/examples/stocks2/lib/stock_arrow.dart
@@ -12,10 +12,11 @@ import 'dart:math' as math;
 import 'dart:sky' as sky;
 
 class StockArrow extends Component {
-  double percentChange;
 
   StockArrow({ Object key, this.percentChange }) : super(key: key);
 
+  final double percentChange;
+
   int _colorIndexForPercentChange(double percentChange) {
     double maxPercent = 10.0;
     double normalizedPercentChange = math.min(percentChange.abs(), maxPercent) / maxPercent;
@@ -71,4 +72,5 @@ class StockArrow extends Component {
         height: size,
         margin: const EdgeDims.symmetric(horizontal: 5.0));
   }
+
 }