Port Sky widgets demo to Dart

sky/engine/core/dom/Element.idl

 // 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.
 
 [
   CustomConstructor,
 ] interface Element : ParentNode {
   readonly attribute DOMString tagName;
 
   boolean hasAttribute(DOMString name);
   [TreatReturnedNullStringAs=Null] DOMString getAttribute(DOMString name);
-  [CustomElementCallbacks, RaisesException] void setAttribute(DOMString name, optional DOMString value);
+  [CustomElementCallbacks, RaisesException] void setAttribute(DOMString name, optional DOMString? value);
   [CustomElementCallbacks] void removeAttribute(DOMString name);
 
   sequence<Attr> getAttributes();
 
   readonly attribute ShadowRoot shadowRoot;
 
   // TODO(abarth): Move to Node.
   readonly attribute CSSStyleDeclaration style;
 
   // TODO(abarth): Remove these when we implement more of the system.
   [RaisesException] boolean matches(DOMString selectors);
   void focus();
   void blur();
   attribute long tabIndex;
   readonly attribute DOMTokenList classList;
   [RaisesException] ShadowRoot ensureShadowRoot();
 
   ClientRect getBoundingClientRect();
   readonly attribute long offsetLeft;
   readonly attribute long offsetTop;
   readonly attribute long offsetWidth;
   readonly attribute long offsetHeight;
   readonly attribute Element offsetParent;
   readonly attribute long clientLeft;
   readonly attribute long clientTop;
   readonly attribute long clientWidth;
   readonly attribute long clientHeight;
 };