Make the popup menu animation correct

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 3f2bab6ff2fbc7b1bc2b5edfdedf5600e67336c0..f78b253dc6655346aed4c9b9e6884c37cf7471fc 100644
--- a/sky/examples/stocks2/lib/stock_arrow.dart
+++ b/sky/examples/stocks2/lib/stock_arrow.dart
@@ -32,12 +32,12 @@ class StockArrow extends Component {
   UINode build() {
     // TODO(jackson): This should change colors with the theme
     Color color = _colorForPercentChange(percentChange);
-    const double size = 40.0;
-    var arrow = new CustomPaint(callback: (sky.Canvas canvas) {
+    const double kSize = 40.0;
+    var arrow = new CustomPaint(callback: (sky.Canvas canvas, Size size) {
       Paint paint = new Paint()..color = color;
       paint.strokeWidth = 1.0;
       var padding = paint.strokeWidth * 3.0;
-      var r = size / 2.0 - padding;
+      var r = kSize / 2.0 - padding;
       canvas.save();
       canvas.translate(padding, padding);
 
@@ -68,8 +68,8 @@ class StockArrow extends Component {
 
     return new Container(
         child: arrow,
-        width: size,
-        height: size,
+        width: kSize,
+        height: kSize,
         margin: const EdgeDims.symmetric(horizontal: 5.0));
   }