[New Multicolumn] A renderer can't be both a parent and a sibling of another renderer.

Source/core/rendering/RenderBlockFlow.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 1960 matching lines @@
 }
 
 void RenderBlockFlow::setStaticInlinePositionForChild(RenderBox* child, LayoutUnit inlinePosition)
 {
     child->layer()->setStaticInlinePosition(inlinePosition);
 }
 
 void RenderBlockFlow::addChild(RenderObject* newChild, RenderObject* beforeChild)
 {
     if (RenderMultiColumnFlowThread* flowThread = multiColumnFlowThread()) {
+        if (beforeChild == flowThread)
+            beforeChild = flowThread->firstChild();
+        ASSERT(!beforeChild || beforeChild->isDescendantOf(flowThread));
         flowThread->addChild(newChild, beforeChild);
         return;
     }
     RenderBlock::addChild(newChild, beforeChild);
 }
 
 void RenderBlockFlow::moveAllChildrenIncludingFloatsTo(RenderBlock* toBlock, bool fullRemoveInsert)
 {
     RenderBlockFlow* toBlockFlow = toRenderBlockFlow(toBlock);
     moveAllChildrenTo(toBlockFlow, fullRemoveInsert);
@@ skipping 914 matching lines @@
 RenderBlockFlow::RenderBlockFlowRareData& RenderBlockFlow::ensureRareData()
 {
     if (m_rareData)
         return *m_rareData;
 
     m_rareData = adoptPtrWillBeNoop(new RenderBlockFlowRareData(this));
     return *m_rareData;
 }
 
 } // namespace blink