Introduce RenderProxyBox and RenderSizedBox

sky/engine/core/painting/Rect.dart

Index: sky/engine/core/painting/Rect.dart
diff --git a/sky/engine/core/painting/Rect.dart b/sky/engine/core/painting/Rect.dart
index 8fdff2ef102c1c887ffc52135232a109d22e248b..b0b7f7c40a0ffc78e7ea6aa061eb4368a8ba3d74 100644
--- a/sky/engine/core/painting/Rect.dart
+++ b/sky/engine/core/painting/Rect.dart
@@ -29,7 +29,11 @@ class Rect {
   }
 
   Rect.fromLTRB(double left, double top, double right, double bottom) {
-    setLTRB(left, top, right, bottom);
+    _value
+      ..[0] = left
+      ..[1] = top
+      ..[2] = right
+      ..[3] = bottom;
   }
 
   Point get upperLeft => new Point(left, top);
@@ -43,14 +47,6 @@ class Rect {
   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) {