Add a function to Offset to get a new offset that is the old offset translated by some given values.

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 translate(double translateX, double translateY) => new Offset(dx + translateX, dy + translateY);
 
   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);
   Offset operator *(double operand) => new Offset(dx * operand, dy * operand);
   Offset operator /(double operand) => new Offset(dx / operand, dy / operand);
   Offset operator ~/(double operand) => new Offset((dx ~/ operand).toDouble(), (dy ~/ operand).toDouble());
   Offset operator %(double operand) => new Offset(dx % operand, dy % operand);
   Rect operator &(Size other) => new Rect.fromLTWH(dx, dy, other.width, other.height);
 
   // does the equivalent of "return new Point(0,0) + this"
   Point toPoint() => new Point(this.dx, this.dy);
 
   String toString() => "Offset($dx, $dy)";
 }