Allow Object when doing lookups.

sdk/lib/html/dart2js/html_dart2js.dart

 /// The Dart HTML library.
 library dart.dom.html;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_collection-dev' hide Symbol;
 import 'dart:html_common';
 import 'dart:indexed_db';
 import 'dart:isolate';
 import 'dart:json' as json;
@@ skipping 7606 matching lines @@
 
 class _ChildrenElementList extends ListBase<Element> {
   // Raw Element.
   final Element _element;
   final HtmlCollection _childElements;
 
   _ChildrenElementList._wrap(Element element)
     : _childElements = element.$dom_children,
       _element = element;
 
-  bool contains(Element element) => _childElements.contains(element);
+  bool contains(Object element) => _childElements.contains(element);
 
 
   bool get isEmpty {
     return _element.$dom_firstElementChild == null;
   }
 
   int get length {
     return _childElements.length;
   }
 
@@ skipping 21959 matching lines @@
       _position = nextPosition;
       return true;
     }
     _current = null;
     _position = _array.length;
     return false;
   }
 
   T get current => _current;
 }