class Gem::Source

def cache_dir(uri)
  # Correct for windows paths
  escaped_path = uri.path.sub(/^\/([a-z]):\//i, '/\\1-/')
  escaped_path.tap(&Gem::UNTAINT)
  File.join Gem.spec_cache_dir, "#{uri.host}%#{uri.port}", File.dirname(escaped_path)
end