function setCallback(shim, promiseMode) {
  return promiseMode ? null : shim.LAST
}
Helper to set the appropriate value of the callback property
in the spec. If it's a promise set to null otherwise set it to `shim.LAST`

@param {Shim} shim instance of shim
@param {boolean} promiseMode is this promise based?
@returns {string|null} appropriate value