function filterKey(contentState: ContentState, entityKey: ?string): ?string {
  if (notEmptyKey(entityKey)) {
    const entity = contentState.getEntity(entityKey);
    return entity.getMutability() === 'MUTABLE' ? entityKey : null;
  }
  return null;
}
Determine whether an entity key corresponds to a `MUTABLE` entity. If so,
return it. If not, return null.