def keys
      if :hash == self.type
        self.children.map { |child| child.children[0] }
      else
        raise Synvert::Core::MethodNotSupported.new "keys is not handled for #{self.debug_info}"
      end
    end
def keys
      if :hash == self.type
        self.children.map { |child| child.children[0] }
      else
        raise Synvert::Core::MethodNotSupported.new "keys is not handled for #{self.debug_info}"
      end
    end
Get keys node of :hash node.

 @return [Array<Parser::AST::Node>] keys node.
 @raise [Synvert::Core::MethodNotSupported] if calls on other node.