getInstance static method
Returns the canonicalized instance of the given reflector type.
If type is not a subclass of Reflectable, or if it is such a class
but no entities are covered (that is, it is unused, so we don't have
any reflection data for it) then null is returned.
Implementation
static Reflectable? getInstance(Type type) {
  for (Reflectable reflector in implementation.reflectors) {
    if (reflector.runtimeType == type) return reflector;
  }
  return null;
}