prefer_const_constructors_in_immutables

Constructors in '@immutable' classes should be declared as 'const'.

The analyzer produces this diagnostic when a non-const constructor is found in a class that has the @immutable annotation.

The following code produces this diagnostic because the constructor in C isn't declared as const even though C has the @immutable annotation:

import 'package:meta/meta.dart';
​
@immutable
class C {
  final int f;
​
  C(this.f);
}

If the class really is intended to be immutable, then add the const modifier to the constructor:

import 'package:meta/meta.dart';
​
@immutable
class C {
  final int f;
​
  const C(this.f);
}

If the class is mutable, then remove the @immutable annotation:

class C {
  final int f;
​
  C(this.f);
}