MapValueSet<K, V>  constructor 
- Map<K, V> _baseMap,
- K _keyForValue(- V
 
Creates a new MapValueSet based on _baseMap.
_keyForValue returns the key in the map that should be associated with
the given value. The set's notion of equality is identical to the equality
of the return values of _keyForValue.
Implementation
MapValueSet(this._baseMap, this._keyForValue);