Add dummy implementation of defaultResponseHeaders

This fixes the analyzer redness on the buildbot.

TBR=nweiz@google.com
BUG=

Review URL: https://codereview.chromium.org//460973002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@39122 260f80e4-7a28-3924-810f-c04153c831b5

pkgs/http_multi_server/lib/http_multi_server.dart

diff --git a/pkgs/http_multi_server/lib/http_multi_server.dart b/pkgs/http_multi_server/lib/http_multi_server.dart
index c055367..76c3eba 100644
--- a/pkgs/http_multi_server/lib/http_multi_server.dart
+++ b/pkgs/http_multi_server/lib/http_multi_server.dart
@@ -27,6 +27,9 @@
     }
   }
 
+  HttpHeaders get defaultResponseHeaders =>
+      throw new UnsupportedError('defaultResponseHeaders not supported');
+
   Duration get idleTimeout => _servers.first.idleTimeout;
   set idleTimeout(Duration value) {
     for (var server in _servers) {