Split Size into Size and Offset.

sky/engine/core/painting/Point.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 part of dart.sky;
 
 /// Holds 2 floating-point coordinates.
 class Point {
   const Point(this.x, this.y);
 
   final double x;
   final double y;
 
   static const Point origin = const Point(0.0, 0.0);
 
   bool operator ==(other) => other is Point && x == other.x && y == other.y;
-  Size operator -(Point other) => new Size(x - other.x, y - other.y);
-  Point operator +(Size size) => new Point(x + size.width, y + size.height);
+  Point operator -() => new Point(-x, -y);
+  Offset operator -(Point other) => new Offset(x - other.x, y - other.y);
+  Point operator +(Offset other) => new Point(x + other.dx, y + other.dy);
 
-  // does the equivalent of "return this - Point(0,0)"
-  Size toSize() => new Size(x, y);
+  // does the equivalent of "return this - Point.origin"
+  Offset toOffset() => new Offset(x, y);
 
   int get hashCode {
     int result = 373;
     result = 37 * result + x.hashCode;
     result = 37 * result + y.hashCode;
     return result;
   }
   String toString() => "Point($x, $y)";
 }