Fix pub test that broke with changes in dart:convert

(dart:convert no longer escapes '/' since https://code.google.com/p/dart/source/detail?r=45003)

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1069413003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@45017 260f80e4-7a28-3924-810f-c04153c831b5

test/serve/404_page_test.dart

@@ -39,7 +39,7 @@ main() {
       expect(response.statusCode, equals(404));
 
       // Should mention the asset that can't be found.
-      expect(response.body, contains('"/packages"'));
+      expect(response.body, contains('"/packages"'));
     });
 
     endPubServe();