must_be_immutable

This class (or a class that this class inherits from) is marked as '@immutable', but one or more of its instance fields aren't final: {0}

The analyzer produces this diagnostic when an immutable class defines one or more instance fields that aren't final. A class is immutable if it's marked as being immutable using the annotation immutable or if it's a subclass of an immutable class.

The following code produces this diagnostic because the field x isn't final:

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

If instances of the class should be immutable, then add the keyword final to all non-final field declarations:

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

If the instances of the class should be mutable, then remove the annotation, or choose a different superclass if the annotation is inherited:

class C {
  int x;
​
  C(this.x);
}