title: why doesn't Dictionary<TKey, TValue>.KeyCollection Class have its own Contains method?
body: <p><code>Dictionary&lt;TKey, TValue&gt;.KeyCollection Class</code> implements the <code>contains</code> through extension method from <code>IEnumerable</code>, which is O(n)</p>
  
  <p>why doesn't it have a native one with O(1)? it's kind of HashSet isn't it?</p>