Fixing Node.clone not returning upgraded custom elements.

tools/dom/idl/dart/dart.idl

 // This file introduces / supplements and forces Dart declarations.
 
 [Supplemental,
  Constructor]
 interface AudioContext {
   // TODO(ager): Auto-generate this custom method when the info about retaining
   // typed arrays is in the IDL.
   [Custom] void decodeAudioData(ArrayBuffer audioData, AudioBufferCallback successCallback, AudioBufferCallback errorCallback);
 };
 
@@ skipping 22 matching lines @@
   [Suppressed] DOMObject getCSSCanvasContext(DOMString contextId, DOMString name, long width, long height);
   CanvasRenderingContext getCSSCanvasContext(DOMString contextId, DOMString name, long width, long height);
   [Custom] Element createElement(DOMString tagName);
   [Custom] Element createElement(DOMString localName, DOMString typeExtension);
   [Custom] Element createElementNS(DOMString namespaceURI, DOMString qualifiedName);
   [Custom] Element createElementNS(DOMString namespaceURI, DOMString qualifiedName, DOMString typeExtension);
 };
 
 [Supplemental]
 interface Node {
-  Node cloneNode([Default=Undefined] optional boolean deep);
+  [Custom] Node cloneNode([Default=Undefined] optional boolean deep);
 };
 
 [Supplemental]
 interface ScriptProcessorNode {
   [Suppressed] attribute EventListener onaudioprocess;
   [Custom] void _setEventListener(EventListener eventListener);
 };
 
 /*
 // TODO(vsm): How should this interact with the new ChildNode and
@@ skipping 290 matching lines @@
 // TextEncoder/TextDecoder
 [Suppressed]
 interface TextEncoder {};
 
 [Suppressed]
 interface TextDecoder {};
 
 [Supplemental]
 interface Window : EventTarget {};