prefer_is_not_operator

Use the 'is!' operator rather than negating the value of the 'is' operator.

The analyzer produces this diagnostic when the prefix ! operator is used to negate the result of an is test.

The following code produces this diagnostic because the result of testing to see whether o is a String is negated using the prefix ! operator:

String f(Object o) {
  if (!(o is String)) {
    return o.toString();
  }
  return o;
}

Use the is! operator instead:

String f(Object o) {
  if (o is! String) {
    return o.toString();
  }
  return o;
}

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