class Bundler::Source::Rubygems

def fetch_gem(spec, previous_spec = nil)
  spec.fetch_platform
  cache_path = download_cache_path(spec) || default_cache_path_for(rubygems_dir)
  gem_path = package_path(cache_path, spec)
  return gem_path if File.exist?(gem_path)
  SharedHelpers.filesystem_access(cache_path) do |p|
    FileUtils.mkdir_p(p)
  end
  download_gem(spec, cache_path, previous_spec)
  gem_path
end