Use Point, Size, and Rect in layout2.dart

sky/engine/core/painting/Rect.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 4 floating-point coordinates for a rectangle.
 class Rect {
   final Float32List _value = new Float32List(4);
   double get left => _value[0];
   double get top => _value[1];
   double get right => _value[2];
   double get bottom => _value[3];
 
   Rect();
 
   Rect.fromPointAndSize(Point point, Size size) {
     _value
       ..[0] = point.x
       ..[1] = point.y
       ..[2] = point.x + size.width
       ..[3] = point.y + size.height;
   }
 
+  Rect.fromSize(Size size) {
+    _value
+      ..[2] = size.width
+      ..[3] = size.height;
+  }
+
   Rect.fromLTRB(double left, double top, double right, double bottom) {
     setLTRB(left, top, right, bottom);
   }
 
   Point get upperLeft => new Point(left, top);
   Point get lowerRight => new Point(right, bottom);
+  Point get center => new Point(left + right / 2.0, top + bottom / 2.0);
 
   Size get size => new Size(right - left, bottom - top);
 
   // Rects are inclusive of the top and left edges but exclusive of the bottom
   // right edges.
   bool contains(Point point) =>
       point.x >= left && point.x < right && point.y >= top && point.y < bottom;
 
   void setLTRB(double left, double top, double right, double bottom) {
     _value
       ..[0] = left
       ..[1] = top
       ..[2] = right
       ..[3] = bottom;
   }
 
   bool operator ==(other) {
     if (!(other is Rect)) return false;
     for (var i = 0; i < 4; ++i) {
       if (_value[i] != other._value[i]) return false;
     }
     return true;
   }
   int get hashCode {
     return _value.fold(373, (value, item) => (37 * value + item.hashCode));
   }
   String toString() => "Rect.LTRB($left, $top, $right, $bottom)";
 }