Verify that callers correctly set parentUsesSize if they use the child's size (and fix a few cases …

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 ed02a664f919c2564764ca5c52a2105bfc83c89d..fefdf4f329d8e244c8604d011bb9945c7fc97428 100644
--- a/sky/engine/core/painting/Size.dart
+++ b/sky/engine/core/painting/Size.dart
@@ -8,6 +8,7 @@ part of dart.sky;
 /// Think of this as a vector from Point(0,0) to Point(size.width, size.height)
 class Size {
   const Size(this.width, this.height);
+  Size.copy(Size source) : width = source.width, height = source.height;
   const Size.fromWidth(this.width) : height = double.INFINITY;
   const Size.fromHeight(this.height) : width = double.INFINITY;