[html5lib] fix another type annotation

Review URL: https://codereview.chromium.org//278693002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/third_party/html5lib@35948 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/test/support.dart b/test/support.dart
index d889158..0118ecf 100644
--- a/test/support.dart
+++ b/test/support.dart
@@ -90,7 +90,7 @@
 }
 
 /// Serialize the [document] into the html5 test data format.
-testSerializer(Document document) {
+testSerializer(document) {
   return (new TestSerializer()..visit(document)).toString();
 }