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
1 file changed
tree: 255fe659d0468130672b28148f1a1f82babe0143
  1. pkgs/