Split Size into Size and Offset.

sky/engine/core/painting/Offset.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 a 2D floating-point offset.
+/// Think of this as a vector from an unspecified point
+class Offset extends OffsetBase {
+  const Offset(dx, dy) : super(dx, dy);
+  Offset.copy(Offset source) : super(source.dx, source.dy);
+
+  double get dx => _dx;
+  double get dy => _dy;
+
+  static const Offset zero = const Offset(0.0, 0.0);
+  static const Offset infinite = const Offset(double.INFINITY, double.INFINITY);
+
+  Offset scale(double scaleX, double scaleY) => new Offset(dx * scaleX, dy * scaleY);
+
+  Offset operator -() => new Offset(-dx, -dy);
+  Offset operator -(Offset other) => new Offset(dx - other.dx, dy - other.dy);
+  Offset operator +(Offset other) => new Offset(dx + other.dx, dy + other.dy);
+
+  // does the equivalent of "return new Point(0,0) + this"
+  Point toPoint() => new Point(this.dx, this.dy);
+
+  String toString() => "Offset($dx, $dy)";
+}