Adds all key-value pairs of other to this map.
If a key of other is already in this map, its value is overwritten.
The operation is equivalent to doing this[key] = value for each key
and associated value in other. It iterates over other, which must
therefore not change during the iteration.
Source
void addAll(Map<String, String> other) {
other.forEach((k, v) { this[k] = v; });
}