Use Point, Size, and Rect in layout2.dart

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 5697086c319c9361d59a5c736e3956d5e571b96e..aa65274cd5e38330ca1f381784aaf2a51af5be47 100644
--- a/sky/engine/core/painting/Size.dart
+++ b/sky/engine/core/painting/Size.dart
@@ -11,6 +11,8 @@ class Size {
 
   Size(this.width, this.height);
 
+  Size.infinite() : width = double.INFINITY, height = double.INFINITY;
+
   bool operator ==(other) {
     if (!(other is Size)) return false;
     return width == other.width && height == other.height;