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In Dart, multiple constructors can be implemented using the factory keyword.

For example, suppose we have a class named Person with the following properties:

class Person {
  String name;
  int age;

  Person(this.name, this.age);
}

We can add another constructor that accepts only the name of the person and sets the age to a default value of 18. To do this, we use the factory keyword and create a named constructor:

class Person {
  String name;
  int age;

  Person(this.name, this.age);

  factory Person.fromName(String name) {
    return Person(name, 18);
  }
}

Now, we can create a new Person object with just the name parameter by calling the named constructor like this:

var john = Person.fromName('John');

This will create a new Person object with the name "John" and age 18.

Note that the fromName constructor is a factory constructor because it does not create a new instance of the class directly, but instead returns an existing instance created within the constructor using the return keyword.