lib/mock.js
function decorateFn (fn) {
  fn.returns = function (value) {
    fn._returns = value
    return fn
  }
  return fn
}
function decorateFn (fn) {
  fn.returns = function (value) {
    fn._returns = value
    return fn
  }
  return fn
}
Decorate a mock function with chainable methods.