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

lib/html/templates/html/impl/impl_NodeList.darttemplate

Index: lib/html/templates/html/impl/impl_NodeList.darttemplate
diff --git a/lib/html/templates/html/impl/impl_NodeList.darttemplate b/lib/html/templates/html/impl/impl_NodeList.darttemplate
index 4e0f698837d5d22e19a5a9b32a08b3322f1e4ef1..7a42e98ceb4075ec6cb723ebbeb5032a6a2d7148 100644
--- a/lib/html/templates/html/impl/impl_NodeList.darttemplate
+++ b/lib/html/templates/html/impl/impl_NodeList.darttemplate
@@ -11,6 +11,8 @@ class _ListWrapper<E> implements List<E> {
 
   Iterator<E> iterator() => _list.iterator();
 
+  bool contains(E element) => _list.contains(element);
+
   void forEach(void f(E element)) => _list.forEach(f);
 
   Collection map(f(E element)) => _list.map(f);
@@ -126,6 +128,8 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
     _parent.$dom_replaceChild(value, this[index]);
   }
 
+  bool contains(Node element) => _Collections.contains(this, element);
+
   void forEach(void f(Node element)) => _Collections.forEach(this, f);
 
   Collection map(f(Node element)) => _Collections.map(this, [], f);