title: System.Collections.Specialized.HybridDictionary missing in Xamarin
body: <p><br>
  I want to implement functionality using HybridDictionary in Xamarin Forms.  I can add reference of System.Collections.Specialized but its missing  HybridDictionary class definition. I can see Only <br></p>
  
  <p>INotifyCollectionChanged <br>
  NotifyCollectionChangedAction <br>
  NotifyCollectionChangedEventArgs <br>
  NotifyCollectionChangedEventHandler</p>
  
  <p>I can  use other collections except this. Am I missing any thing to add?</p>