Make Attr.localName/name and Element.localName/tagName not nullable

Source/core/dom/Element.idl

 /*
  * Copyright (C) 2006, 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * 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,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 // https://dom.spec.whatwg.org/#interface-element
 
 [
     TypeChecking=Interface,
 ] interface Element : Node {
     readonly attribute DOMString? namespaceURI;
     readonly attribute DOMString? prefix;
-    // FIXME: localName/tagName should not be nullable.
-    readonly attribute DOMString? localName;
-    readonly attribute DOMString? tagName;
+    readonly attribute DOMString localName;
+    readonly attribute DOMString tagName;
 
     [Reflect, ExposeJSAccessors] attribute DOMString id;
     [Reflect=class] attribute DOMString className;
     [SameObject, PerWorldBindings] readonly attribute DOMTokenList classList;
 
     [MeasureAs=HasAttributes] boolean hasAttributes();
     [SameObject, PerWorldBindings, ImplementedAs=attributesForBindings] readonly attribute NamedNodeMap attributes;
     DOMString? getAttribute(DOMString name);
     DOMString? getAttributeNS(DOMString? namespaceURI, DOMString localName);
     [RaisesException, CustomElementCallbacks] void setAttribute(DOMString name, DOMString value);
@@ skipping 101 matching lines @@
     [RuntimeEnabled=Touch] attribute EventHandler ontouchcancel;
     [RuntimeEnabled=Touch] attribute EventHandler ontouchend;
     [RuntimeEnabled=Touch] attribute EventHandler ontouchmove;
     [RuntimeEnabled=Touch] attribute EventHandler ontouchstart;
     attribute EventHandler onwheel;
 };
 
 Element implements ParentNode;
 Element implements ChildNode;
 Element implements NonDocumentTypeChildNode;