Can you explain what the keyword 'open' means in the Dart programming language?

In the Dart language, the keyword open is used to denote a class that can be inherited by other classes.

By default, in Dart, all classes are considered "closed" for inheritance (final), unless explicitly specified otherwise. Using open allows making a class open for extension, meaning other classes can inherit from it.

Example:

open class Animal {
  void speak() {
    print('Animal sound');
  }
}

class Dog extends Animal {
  @override
  void speak() {
    print('Bark');
  }
}

void main() {
  Dog dog = Dog();
  dog.speak(); // Outputs: Bark
}

Thus, open is a way to control inheritance and extendability of classes in Dart.