Use Point, Size, and Rect in layout2.dart

sky/engine/core/painting/Size.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 size.
 class Size {
   double width;
   double height;
 
   Size(this.width, this.height);
 
+  Size.infinite() : width = double.INFINITY, height = double.INFINITY;
+
   bool operator ==(other) {
     if (!(other is Size)) return false;
     return width == other.width && height == other.height;
   }
   int get hashCode {
     int result = 373;
     result = 37 * result + width.hashCode;
     result = 37 * result + height.hashCode;
     return result;
   }
   String toString() => "Size($width, $height)";
 }