Assert that you can't mutate the tree during paint or layout.

sky/sdk/lib/framework/rendering/object.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 '../node.dart';
 import '../scheduler.dart' as scheduler;
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 
 class ParentData {
@@ skipping 28 matching lines @@
 abstract class RenderObject extends AbstractNode {
 
   // LAYOUT
 
   // parentData is only for use by the RenderObject that actually lays this
   // node out, and any other nodes who happen to know exactly what
   // kind of node that is.
   ParentData parentData;
   void setParentData(RenderObject child) {
     // override this to setup .parentData correctly for your class
+    assert(!_debugDoingLayout);
+    assert(!debugDoingPaint);
     if (child.parentData is! ParentData)
       child.parentData = new ParentData();
   }
 
   void adoptChild(RenderObject child) { // only for use by subclasses
     // call this whenever you decide a node is a child
+    assert(!_debugDoingLayout);
+    assert(!debugDoingPaint);
     assert(child != null);
     setParentData(child);
     super.adoptChild(child);
     markNeedsLayout();
   }
   void dropChild(RenderObject child) { // only for use by subclasses
+    assert(!_debugDoingLayout);
+    assert(!debugDoingPaint);
     assert(child != null);
     assert(child.parentData != null);
     child.parentData.detach();
     if (child._relayoutSubtreeRoot != child) {
       child._relayoutSubtreeRoot = null;
       child._needsLayout = true;
     }
     super.dropChild(child);
     markNeedsLayout();
   }
@@ skipping 318 matching lines @@
     }
   }
 
   ChildType get firstChild => _firstChild;
   ChildType get lastChild => _lastChild;
   ChildType childAfter(ChildType child) {
     assert(child.parentData is ParentDataType);
     return child.parentData.nextSibling;
   }
 }