Fixed Native custom element

sdk/lib/html/html_common/conversions_dartium.dart

 part of html_common;
 
 convertDartToNative_PrepareForStructuredClone(value) =>
     new _StructuredCloneDartium().convertDartToNative_PrepareForStructuredClone(value);
 
 convertNativeToDart_AcceptStructuredClone(object, {mustCopy: false}) =>
     new _AcceptStructuredCloneDartium().convertNativeToDart_AcceptStructuredClone(object, mustCopy: mustCopy);
 
 class _StructuredCloneDartium extends _StructuredClone {
   newJsMap() => new js.JsObject(js.context["Object"]);
@@ skipping 158 matching lines @@
     var defaultNS = jsObject['namespaceURI'] == 'http://www.w3.org/1999/xhtml' ||
         jsObject['namespaceURI'] ==  'http://www.w3.org/2000/svg';
     if (customElementClass != null && extendsTag == "" && defaultNS) {
       // The customElementClass is known but we can't create the real class so
       // create the HtmlElement and it will get upgraded when registerElement's
       // createdCallback is called.
       func = getHtmlCreateFunction('HTMLElement');
     } else {
       func = getHtmlCreateFunction(jsTypeName);
       if (func == null) {
-        if (jsTypeName == 'auto-binding') {
-          func = getHtmlCreateFunction('HTMLTemplateElement');
-        } else if (jsObject.toString() == "[object HTMLElement]") {
-          // One last ditch effort could be a JS custom element.
-          func = getHtmlCreateFunction('HTMLElement');
+        // Start walking the prototype chain looking for a JS class.
+        var prototype = jsObject['__proto__'];
+        var keepWalking = true;
+        while (keepWalking && prototype.hasProperty('__proto__')) {
+          prototype = prototype['__proto__'];
+          if (prototype != null && prototype is Element &&
+              prototype.blink_jsObject != null) {
+            // We're a Dart class that's pointing to a JS class.
+            var blinkJso = prototype.blink_jsObject;
+            jsTypeName = blinkJso['constructor']['name'];
+            func = getHtmlCreateFunction(jsTypeName);
+            keepWalking = func == null;
+          }
         }
       }
     }
 
     // Can we construct a Dart class?
     if (func != null) {
       dartClass_instance = func();
 
       // Wrap our Dart instance in both directions.
       dartClass_instance.blink_jsObject = jsObject;
@@ skipping 178 matching lines @@
 
   final js.JsArray blink_jsObject;
 
   operator [](int index) => wrap_jso(js.JsNative.getArrayIndex(blink_jsObject, index));
 
   operator []=(int index, value) => blink_jsObject[index] = value;
 
   int get length => blink_jsObject.length;
   int set length(int newLength) => blink_jsObject.length = newLength;
 }