class Opal::Builder

def tree_requires(asset, asset_path)
  dirname = asset_path.to_s.empty? ? Pathname.pwd : Pathname(asset_path).expand_path.dirname
  abs_base_paths = path_reader.paths.map { |p| File.expand_path(p) }
  asset.required_trees.flat_map do |tree|
    abs_tree_path = dirname.join(tree).expand_path.to_s
    abs_base_path = abs_base_paths.find { |p| abs_tree_path.start_with?(p) }
    if abs_base_path
      abs_base_path = Pathname(abs_base_path)
      entries_glob  = Pathname(abs_tree_path).join('**', "*{.js,}.{#{extensions.join ','}}")
      Pathname.glob(entries_glob).map { |file| file.relative_path_from(abs_base_path).to_s }
    else
      [] # the tree is not part of any known base path
    end
  end
end