class Bundler::Source::Rubygems

def cached_built_in_gem(spec)
  cached_path = cached_path(spec)
  if cached_path.nil?
    remote_spec = remote_specs.search(spec).first
    if remote_spec
      cached_path = fetch_gem(remote_spec)
    else
      Bundler.ui.warn "#{spec.full_name} is built in to Ruby, and can't be cached because your Gemfile doesn't have any sources that contain it."
    end
  end
  cached_path
end