cached_built_in_gem

# File lib/bundler/source/rubygems.rb, line 207
      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