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()) {
  if (key == null) throw new ArgumentError(key);
  int comp = _splay(key);
  if (comp == 0) {
    return _root.value;
  }
  int modificationCount = _modificationCount;
  int splayCount = _splayCount;
  V value = ifAbsent();
  if (modificationCount != _modificationCount) {
    throw new ConcurrentModificationError(this);
  }
  if (splayCount != _splayCount) {
    comp = _splay(key);
    // Key is still not there, otherwise _modificationCount would be changed.
    assert(comp != 0);
  }
  _addNewRoot(new _SplayTreeMapNode(key, value), comp);
  return value;
}