invalid_use_of_type_outside_library

The class '{0}' can't be extended outside of its library because it's a final class.

The class '{0}' can't be extended outside of its library because it's an interface class.

The class '{0}' can't be extended, implemented, or mixed in outside of its library because it's a sealed class.

The class '{0}' can't be implemented outside of its library because it's a base class.

The class '{0}' can't be implemented outside of its library because it's a final class.

The class '{0}' can't be used as a mixin superclass constraint outside of its library because it's a final class.

The mixin '{0}' can't be implemented outside of its library because it's a base mixin.

The analyzer produces this diagnostic when an extends, implements, with, or on clause uses a class or mixin in a way that isn't allowed given the modifiers on that class or mixin's declaration.

The message specifies how the declaration is being used and why it isn't allowed.

Given a file a.dart that defines a base class A:

base class A {}

The following code produces this diagnostic because the class B implements the class A, but the base modifier prevents A from being implemented outside of the library where it's defined:

import 'a.dart';
​
final class B implements A {}

Use of this type is restricted outside of its declaring library. If a different, unrestricted type is available that can provide similar functionality, then replace the type:

class B implements C {}
class C {}

If there isn't a different type that would be appropriate, then remove the type, and possibly the whole clause:

class B {}