Check out example codes for "dart he default constructor is already defined. Try giving one of the constructors a name.". It will help you in understanding the concepts better.

Code Example 1

In dart you can not have the same method/constructor name used several times 
(even with different parameters).

In your case you can either use named constructor to define 2 constructors :

class FirstClass {
  FirstClass() {}
  FirstClass.withA(a) {}
}

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