contains method Null safety

Validator contains(
  1. String string,
  2. {String message = "Validator.contains"}
)

Returns a Validator that accepts a value that contains a match of string.

Always rejects null;

Implementation

Validator contains(
  String string, {
  String message = "Validator.contains",
}) {
  return add(
    (value) => value == null || !value.contains(string) ? message : null,
  );
}