Add asserts to catch potential misuses of the rendering framework.

sky/examples/rendering/sector_layout.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 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/object.dart';
 import 'package:sky/rendering/sky_binding.dart';
@@ skipping 57 matching lines @@
   void setParentData(RenderObject child) {
     if (child.parentData is! SectorParentData)
       child.parentData = new SectorParentData();
   }
 
   SectorDimensions getIntrinsicDimensions(SectorConstraints constraints, double radius) {
     return new SectorDimensions.withConstraints(constraints);
   }
 
   SectorConstraints get constraints => super.constraints;
+  bool debugDoesMeetConstraints() {
+    assert(constraints != null);
+    assert(deltaRadius != null);
+    assert(deltaRadius < double.INFINITY);
+    assert(deltaTheta != null);
+    assert(deltaTheta < double.INFINITY);
+    return constraints.minDeltaRadius <= deltaRadius &&
+           deltaRadius <= math.max(constraints.minDeltaRadius, constraints.maxDeltaRadius) &&
+           constraints.minDeltaTheta <= deltaTheta &&
+           deltaTheta <= math.max(constraints.minDeltaTheta, constraints.maxDeltaTheta);
+  }
   void performResize() {
     // default behaviour for subclasses that have sizedByParent = true
     deltaRadius = constraints.constrainDeltaRadius(0.0);
     deltaTheta = constraints.constrainDeltaTheta(0.0);
   }
   void performLayout() {
     // descendants have to either override performLayout() to set both
     // the dimensions and lay out children, or, set sizedByParent to
     // true so that performResize()'s logic above does its thing.
     assert(sizedByParent);
@@ skipping 432 matching lines @@
   stack.add(new RenderSolidColor(const Color(0xFFFFFF00), desiredDeltaRadius: 20.0));
   stack.add(new RenderSolidColor(const Color(0xFFFF9000), desiredDeltaRadius: 20.0));
   stack.add(new RenderSolidColor(const Color(0xFF00FF00)));
   rootCircle.add(stack);
   return new RenderBoxToRenderSectorAdapter(innerRadius: 50.0, child: rootCircle);
 }
 
 void main() {
   new SkyBinding(root: buildSectorExample());
 }