lineItems static method
returns a list of line items from a json object
Implementation
static List<LineItem> lineItems(dynamic json) {
  if (json == null) {
    return [];
  } else if (json is List) {
    return json.map((e) => LineItem.fromJson(e)).toList();
  } else if (json['edges'] == null) {
    return [];
  }
  return (json['edges'] as List)
      .map((e) => LineItem.fromGraphJson(e))
      .toList();
}