EqualityMap<K, V>.from  constructor 
Creates a map with equality based on equality that contains all
key-value pairs of other.
If other has multiple keys that are equivalent according to equality,
the last one reached during iteration takes precedence.
Implementation
EqualityMap.from(Equality<K> equality, Map<K, V> other)
    : super(LinkedHashMap(
          equals: equality.equals,
          hashCode: equality.hash,
          isValidKey: equality.isValidKey)) {
  addAll(other);
}