Implement WebIDL implements

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,
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     [DeliverCustomElementCallbacks, PerWorldBindings, ActivityLog=AccessForIsolatedWorlds, RaisesException] Element createElementNS([TreatNullAs=NullString] DOMString namespaceURI, DOMString qualifiedName,
                             [TreatNullAs=NullString] DOMString typeExtension);
 
     // Page visibility API.
     readonly attribute DOMString webkitVisibilityState;
     readonly attribute boolean webkitHidden;
 
     // Security Policy API: http://dvcs.w3.org/hg/content-security-policy/raw-file/tip/csp-specification.dev.html#script-interfaces
     [EnabledAtRuntime=experimentalContentSecurityPolicyFeatures] readonly attribute SecurityPolicy securityPolicy;
 
-    // ParentNode interface API
-    readonly attribute HTMLCollection children;
-    readonly attribute Element firstElementChild;
-    readonly attribute Element lastElementChild;
-    readonly attribute unsigned long childElementCount;
-
     readonly attribute HTMLScriptElement currentScript;
 };