[html5lib] fix another type annotation

pkg/third_party/html5lib/test/support.dart

 /// Support code for the tests in this directory.
 library support;
 
 import 'dart:io';
 import 'dart:collection';
 import 'package:path/path.dart' as path;
 import 'package:html5lib/src/treebuilder.dart';
 import 'package:html5lib/dom.dart';
 import 'package:html5lib/dom_parsing.dart';
 
@@ skipping 72 matching lines @@
     data.forEach((key, value) {
       if (value.endsWith("\n")) {
         data[key] = value.substring(0, value.length - 1);
       }
     });
     return data;
   }
 }
 
 /// Serialize the [document] into the html5 test data format.
-testSerializer(Document document) {
+testSerializer(document) {
   return (new TestSerializer()..visit(document)).toString();
 }
 
 /// Serializes the DOM into test format. See [testSerializer].
 class TestSerializer extends TreeVisitor {
   final StringBuffer _str;
   int _indent = 0;
   String _spaces = '';
 
   TestSerializer() : _str = new StringBuffer();
@@ skipping 52 matching lines @@
           key = "${attr.prefix} ${attr.name}";
         }
         _newline();
         _str.write('$key="$v"');
       }
       indent -= 2;
     }
     visitChildren(node);
   }
 }