Which libray is 'StreamImagesourceHandler' in?

I see it is being used in many threads. But I am not able to find what lib it is in? API search is getting my only StreamImageSource class. Or is it deprecated?

It is used by:

1. By @JesseLiberty in [Retrieve Image from Disk](https://forums.xamarin.com/discussion/comment/76204#Comment_76204).
2. By @ChWo in [Video on making custom renderers](https://forums.xamarin.com/discussion/comment/79885#Comment_79885).
3. By @MichaelCook in [Platform Specific Properties](https://forums.xamarin.com/discussion/19542/platform-specific-properties).
4. By @MacMcled in [Save image to app directory in Xamarin.Forms Android](https://forums.xamarin.com/discussion/comment/107219/#Comment_107219).