class Sass::CSS

def expand_commas(root)

but it's necessary to get nesting to work properly.
Yes, this expands the amount of code,

color: blue
baz
color: blue
bar
color: blue
foo

into

color: blue
foo, bar, baz

Transform 
def expand_commas(root)
  root.children.map! do |child|
    next child unless Tree::RuleNode === child && child.rule.include?(',')
    child.rule.split(',').map do |rule|
      node = Tree::RuleNode.new(rule, nil)
      node.children = child.children
      node
    end
  end
  root.children.flatten!
end