class Gem::FakeFetcher

def download spec, source_uri, install_dir = Gem.dir
  name = File.basename spec.cache_file
  path = if Dir.pwd == install_dir then # see fetch_command
           install_dir
         else
           File.join install_dir, "cache"
         end
  path = File.join path, name
  if source_uri =~ /^http/ then
    File.open(path, "wb") do |f|
      f.write fetch_path(File.join(source_uri, "gems", name))
    end
  else
    FileUtils.cp source_uri, path
  end
  path
end