cucumber_em.rb

module Cucumber
  module Ast
    class TreeWalker
      alias_method :visit_features_alias, :visit_features

      def visit_features(features)
        EM.synchrony do
          visit_features_alias features
          EM.stop
        end
      end
    end
  end
end

rspec_em.rb

module RSpec
  module Core
    class ExampleGroup
      class << self
        alias_method :run_alias, :run

        def run(reporter)
          if EM.reactor_running?
            run_alias reporter
          else
            out = nil
            EM.synchrony do
              out = run_alias reporter
              EM.stop
            end

            out
          end
        end
      end
    end
  end
end