How to add a new pair to Map in Dart?

To declare your map in Flutter you probably want final:

final Map<String, int> someMap = {
  "a": 1,
  "b": 2,
};

Then, your update should work:

someMap["c"] = 3;

Finally, the update function has two parameters you need to pass, the first is the key, and the second is a function that itself is given one parameter (the existing value). Example:

someMap.update("a", (value) => value + 100);

If you print the map after all of this you would get:

{a: 101, b: 2, c: 3}