Why lots of API reference are missing in my project?

I started develop a portable project in VS 2017, Xamarin.Core v. 2.0.0.0
I need some element from Xamarin but the intellisense doesn't show them. E.g. I see in Xamarin documentation :

var assembly = typeof(LoadResourceText).GetTypeInfo().Assembly;

The intellisense doesn't throw up GetTypeInfo.

Other example:
By API documentation the Environment contains GetCommandLineArgs method, but I could not find it.

I see tons of Xamarin sample codes in the internet which do not work in my environment because of missing API functions.

Why? Must I update something?