Add asserts to catch potential misuses of the rendering framework.

sky/examples/rendering/sector_layout.dart

Index: sky/examples/rendering/sector_layout.dart
diff --git a/sky/examples/rendering/sector_layout.dart b/sky/examples/rendering/sector_layout.dart
index b17b31b98cee3d9d61ebcfbfed6565fe313be2c4..85dcc41691f603735023bbd517748b90112e78ec 100644
--- a/sky/examples/rendering/sector_layout.dart
+++ b/sky/examples/rendering/sector_layout.dart
@@ -75,6 +75,17 @@ abstract class RenderSector extends RenderObject {
   }
 
   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);