withDefault method
Returns a mutable view of this map that responds with the provided default value, when the caller tries to access a non-existent key.
For example:
final map = {'a': 1}.withDefault(42);
print(map['z']); // 42
print(map.containsKey('z')); // false
Implementation
MapWithDefault<K, V> withDefault(V value) => MapWithDefault(this, value);