Document.body should check interface type and be nullable

Source/core/dom/Document.idl

 /*
  * Copyright (C) 2006, 2007, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006, 2007 Samuel Weinig <sam@webkit.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 98 matching lines @@
     // Moved down from HTMLDocument
              [TreatNullAs=NullString, CustomElementCallbacks] attribute DOMString dir;
              [TreatNullAs=NullString, CustomElementCallbacks] attribute DOMString designMode;
              [TreatNullAs=NullString, CustomElementCallbacks] attribute DOMString title;
     readonly attribute DOMString referrer;
              [TreatNullAs=NullString, RaisesException=Setter] attribute DOMString domain;
     readonly attribute DOMString URL;
 
              [TreatNullAs=NullString, RaisesException] attribute DOMString cookie;
 
-             [RaisesException=Setter, CustomElementCallbacks, PerWorldBindings] attribute HTMLElement body;
+    [RaisesException=Setter, CustomElementCallbacks, PerWorldBindings, TypeChecking=Interface] attribute HTMLElement? body;
 
     readonly attribute HTMLHeadElement head;
     readonly attribute HTMLCollection images;
     readonly attribute HTMLCollection applets;
     readonly attribute HTMLCollection embeds;
     [ImplementedAs=embeds] readonly attribute HTMLCollection plugins;
     readonly attribute HTMLCollection links;
     readonly attribute HTMLCollection forms;
     readonly attribute HTMLCollection scripts;
     readonly attribute HTMLCollection anchors;
@@ skipping 86 matching lines @@
     // TODO(davidben): This is a property so attaching a deprecation warning results in false positives when outputting
     // document in the console. It's possible http://crbug.com/43394 will resolve this.
     [MeasureAs=PrefixedPageVisibility, ImplementedAs=visibilityState] readonly attribute DOMString webkitVisibilityState;
     [MeasureAs=PrefixedPageVisibility, ImplementedAs=hidden] readonly attribute boolean webkitHidden;
 
     readonly attribute HTMLScriptElement currentScript;
 };
 
 Document implements GlobalEventHandlers;
 Document implements ParentNode;