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