Unable to find NameValueCollection in Xamarin.Forms project.

Hi,

Does anybody knows where can I find NameValueCollection in Xamarin.Forms portable project?
Referring to this link I have check for NameValueCollection class under System.Collections.Specialized namespace in System.dll assembly but I couldn't find it.

It's quite wired that under System.Collections.Specialized I only can access to following classes and interfaces, which belong to System.ObjectModel.dll assembly:

INotifyCollectionChanged
NotifyCollectionChangedAction
NotifyCollectionChangedEventArgs
NotifyCollectionChangedEventHandler

Should I add System.dll directly into my portable project?