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

lib/html/templates/html/impl/impl_Node.darttemplate

Index: lib/html/templates/html/impl/impl_Node.darttemplate
diff --git a/lib/html/templates/html/impl/impl_Node.darttemplate b/lib/html/templates/html/impl/impl_Node.darttemplate
index 155673d268037b2b04c82540b10d392ac6f6b3e6..8b369d145dde3881ab2298783d5e70bb761d5c75 100644
--- a/lib/html/templates/html/impl/impl_Node.darttemplate
+++ b/lib/html/templates/html/impl/impl_Node.darttemplate
@@ -56,6 +56,8 @@ $endif
 
   // TODO(jacobr): We can implement these methods much more efficiently by
   // looking up the nodeList only once instead of once per iteration.
+  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);