SplayTreeMap<K, V> constructor

SplayTreeMap<K, V>([int compare(K key1, K key2), bool isValidKey(dynamic potentialKey) ])

Implementation 

SplayTreeMap([int compare(K key1, K key2), bool isValidKey(potentialKey)])
    : _comparator = compare ?? _defaultCompare<K>(),
      _validKey = isValidKey ?? ((v) => v is K);