function assignId(shimName) {
  const id = shimIds.get(shimName)
  this.id = id || makeId()

  if (shimName && !id) {
    shimIds.set(shimName, this.id)
  }
}
Assigns id to shim instance.
If shimName is present it will reuse an id
otherwise it'll create a unique id.

@param {string} shimName Used to persist shim ids across different instances.
@memberof Shim.prototype