Explain what the sealed modifier means in programming and how it is used.

The sealed modifier in programming is used to restrict class inheritance. A class declared as sealed can only have a predefined set of subclasses, usually declared in the same file or module. This allows the compiler and developer to know all possible inheritance options, improving type safety and simplifying the handling of such classes.

In Dart (used in Flutter), starting from version 3.0, the sealed modifier was introduced, enabling the creation of class hierarchies with limited inheritance.

Example of usage in Dart:

sealed class Animal {}

class Dog extends Animal {}
class Cat extends Animal {}

// Attempting to create a class outside this file that inherits from Animal will cause an error.

This is useful, for example, when implementing the "sum types" pattern or when you need to explicitly limit inheritance options for more predictable logic.