The simple solution is to target .NET 4.5, Windows Store apps (Windows 8.1), Xamarin.iOS, and Xamarin.Android (4.5-Profile7). Then you won't need the HttpClient NuGet package to get those APIs.
If Xamarin is crashing when you try to select a 4.5 profile, please make sure you're using the latest version and then report the bug with details about how to repro it and what happens.
Also the HttpClient NuGet package should allow you to use HttpClient from PCL profile 158. Make sure you're using the latest version of NuGet. ".NET Portable Subset" should always appear in your references for a PCL, that's not related to the NuGet package. When you install the HttpClient NuGet package, you should also see references to System.Net.Http and other assemblies in your references.
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