How to run an external script from source control in the Jenkins Build Flow plugin DSL

jenkins-build-flow-external.groovy

//
// Copy this into your Jenkins Build Flow plugin DSL, and call run_workspace_script() with the name of your checked in script
//
// Based on code at http://jorgemanrubia.net/2009/10/10/evaluating-code-dynamically-in-groovy
//
def run_workspace_script(filename) {
  def code = new File(build.workspace.child(filename).toString()).text
  def code_as_closure = "{->${code}}"
  def closure = evaluate(code_as_closure)
  closure.delegate=this
  closure()
}