V putIfAbsent(K key, V ifAbsent())

Look up the value of key, or add a new value if it isn't there.

Returns the value associated to key, if there is one. Otherwise calls ifAbsent to get a new value, associates key to that value, and then returns the new value.

Map<String, int> scores = {'Bob': 36};
for (var key in ['Bob', 'Rohan', 'Sophena']) {
  scores.putIfAbsent(key, () => key.length);
}
scores['Bob'];      // 36
scores['Rohan'];    //  5
scores['Sophena'];  //  7

Calling ifAbsent must not add or remove keys from the map.

Source 

V putIfAbsent(K key, V ifAbsent()) {
  throw new UnsupportedError("Cannot modify unmodifiable map");
}