Creates an unmodifiable hash based map containing the entries of other.
The keys must all be assignable to K and the values to V.
The other map itself can have any type.
The map requires the keys to implement compatible
operator== and hashCode, and it allows null as a key.
The created map iterates keys in a fixed order,
preserving the order provided by other.
The resulting map behaves like the result of Map.from,
except that the map returned by this constructor is not modifiable.
Source
external factory Map.unmodifiable(Map other);