class Sass::Tree::Visitors::Perform

def arg_hash(map)
  Sass::Util.map_keys(map.to_h) do |key|
    next key.value if key.is_a?(Sass::Script::Value::String)
    raise Sass::SyntaxError.new("Variable keyword argument map must have string keys.\n" +
      "#{key.inspect} is not a string in #{map.inspect}.")
  end
end