Map<K, V>.identity constructor
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;