Add asserts to prevent people from setting box.parentData.position or box.size except during layout.

sky/sdk/lib/framework/rendering/box.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.
 
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'dart:typed_data';
 import 'object.dart';
 import '../painting/shadows.dart';
 import 'package:vector_math/vector_math.dart';
@@ skipping 141 matching lines @@
     value = 37 * value + minWidth.hashCode;
     value = 37 * value + maxWidth.hashCode;
     value = 37 * value + minHeight.hashCode;
     value = 37 * value + maxHeight.hashCode;
     return value;
   }
   String toString() => "BoxConstraints($minWidth<=w<$maxWidth, $minHeight<=h<$maxHeight)";
 }
 
 class BoxParentData extends ParentData {
-  Point position = Point.origin;
+  Point _position = Point.origin;
+  Point get position => _position;
+  void set position(Point value) {
+    assert(RenderObject.debugDoingLayout);
+    _position = value;
+  }
   String toString() => 'position=$position';
 }
 
 abstract class RenderBox extends RenderObject {
 
   void setParentData(RenderObject child) {
     if (child.parentData is! BoxParentData)
       child.parentData = new BoxParentData();
   }
 
@@ skipping 38 matching lines @@
     assert(sizedByParent);
   }
 
   bool hitTest(HitTestResult result, { Point position }) {
     hitTestChildren(result, position: position);
     result.add(this);
     return true;
   }
   void hitTestChildren(HitTestResult result, { Point position }) { }
 
-  Size size = Size.zero;
+  Size _size = Size.zero;
+  Size get size => _size;
+  void set size(Size value) {
+    assert(RenderObject.debugDoingLayout);
+    _size = value;
+  }
 
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}size: ${size}\n';
 }
 
 abstract class RenderProxyBox extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
   RenderProxyBox(RenderBox child) {
     this.child = child;
   }
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
@@ skipping 774 matching lines @@
 
   void defaultPaint(RenderObjectDisplayList canvas) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position);
       child = child.parentData.nextSibling;
     }
   }
 }