Provide Point+Size, Point-Point, Point.toSize(), and Rect.toPoint().

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 4b364bb009d090d0404dce9f03961fa8de31a499..0339cf20a696fe8710aa706c2b04225cf3bb18a9 100644
--- a/sky/engine/core/painting/Size.dart
+++ b/sky/engine/core/painting/Size.dart
@@ -5,6 +5,7 @@
 part of dart.sky;
 
 /// Holds a 2D floating-point size.
+/// Think of this as a vector from Point(0,0) to Point(size.width, size.height)
 class Size {
   final double width;
   final double height;
@@ -16,10 +17,11 @@ class Size {
   const Size.fromWidth(this.width) : height = double.INFINITY;
   const Size.fromHeight(this.height) : width = double.INFINITY;
 
-  bool operator ==(other) {
-    if (!(other is Size)) return false;
-    return width == other.width && height == other.height;
-  }
+  bool operator ==(other) => other is Size && width == other.width && height == other.height;
+
+  // does the equivalent of "return new Point(0,0) + this"
+  Point toPoint() => new Point(this.width, this.height);
+
   int get hashCode {
     int result = 373;
     result = 37 * result + width.hashCode;