Responder class
The behavior of a method call in the mock library is specified with Responders. A Responder has a value to throw or return (depending on the type of action), and can either be one-shot, multi-shot, or infinitely repeating, depending on the value of [count (1, greater than 1, or 0 respectively).
class Responder { var value; Action action; int count; Responder(this.value, [this.count = 1, this.action = Action.RETURN]); }