code: function tryFn1 (fn, arg, err) {
    try {
      err.err = null
      return fn(arg)
    } catch (e) {
      err.err = e
    }
  }
docs: minimal try/catch to prevent deopts see https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#2-unsupported-syntax