Map<K, V>.identity constructor Null safety
Creates an identity map with the default implementation, LinkedHashMap.
An identity map uses identical for equality and identityHashCode
for hash codes of keys instead of the intrinsic Object.operator== and
Object.hashCode of the keys.
The returned map allows null as a key.
It iterates in key insertion order.
Implementation
factory Map.identity() = LinkedHashMap<K, V>.identity;