Introduce sky.Sky.zero

sky/engine/core/painting/Size.dart

Index: sky/engine/core/painting/Size.dart
diff --git a/sky/engine/core/painting/Size.dart b/sky/engine/core/painting/Size.dart
index 139fc16db171bcf660afca5f77f5fb3530197b46..b4a1e9fe3c3c3db57e1e9feb1b2bb68eec95299f 100644
--- a/sky/engine/core/painting/Size.dart
+++ b/sky/engine/core/painting/Size.dart
@@ -15,6 +15,7 @@ class Size {
   const Size.fromWidth(this.width) : height = double.INFINITY;
   const Size.fromHeight(this.height) : width = double.INFINITY;
 
+  static const Size zero = const Size(0.0, 0.0);
   static const Size infinite = const Size(double.INFINITY, double.INFINITY);
 
   bool operator ==(other) => other is Size && width == other.width && height == other.height;