void addAll(Map<String, String> other)

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;
  });
}