Generate contains() method for list mixin only if not already defined.

lib/html/templates/html/impl/impl_DocumentFragment.darttemplate

Index: lib/html/templates/html/impl/impl_DocumentFragment.darttemplate
diff --git a/lib/html/templates/html/impl/impl_DocumentFragment.darttemplate b/lib/html/templates/html/impl/impl_DocumentFragment.darttemplate
index 229d58279eed495e8064d137666c864869a30b7b..e8a1e9dc2c1c90e1dff2be723434c3ac2db5ca02 100644
--- a/lib/html/templates/html/impl/impl_DocumentFragment.darttemplate
+++ b/lib/html/templates/html/impl/impl_DocumentFragment.darttemplate
@@ -46,6 +46,10 @@ class _FilteredElementList implements List {
     add(value);
   }
 
+  bool contains(Element element) {
+    return element is Element && _childNodes.contains(element);
+  }
+
   void sort([Comparator<Element> compare = Comparable.compare]) {
     throw const UnsupportedOperationException('TODO(jacobr): should we impl?');
   }