Introduce RenderProxyBox and RenderSizedBox

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 aa65274cd5e38330ca1f381784aaf2a51af5be47..f2d43a65ca1b621bbf42bea715d00ada928bde12 100644
--- a/sky/engine/core/painting/Size.dart
+++ b/sky/engine/core/painting/Size.dart
@@ -6,12 +6,12 @@ part of dart.sky;
 
 /// Holds a 2D floating-point size.
 class Size {
-  double width;
-  double height;
+  final double width;
+  final double height;
 
-  Size(this.width, this.height);
+  const Size(this.width, this.height);
 
-  Size.infinite() : width = double.INFINITY, height = double.INFINITY;
+  const Size.infinite() : width = double.INFINITY, height = double.INFINITY;
 
   bool operator ==(other) {
     if (!(other is Size)) return false;