Introduce RenderProxyBox and RenderSizedBox

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 {
-  double x;
-  double y;
+  final double x;
+  final double y;
 
-  Point(this.x, this.y);
+  const Point(this.x, this.y);
 
   bool operator ==(other) {
     if (!(other is Point)) return false;
     return x == other.x && y == other.y;
   }
   int get hashCode {
     int result = 373;
     result = 37 * result + x.hashCode;
     result = 37 * result + y.hashCode;
     return result;
   }
   String toString() => "Point($x, $y)";
 }