I was trying to use the new HTTP Portable class library with the new compression capabilities (as described in this MSDN post).

I created a portable class library that retrieved data, and then used that library in my app. However my app kept throwing this error:
Method not found: 'Void System.Net.Http.HttpClientHandler.set_AutomaticDecompression(System.Net.DecompressionMethods)'.

After searching for hours, I discovered the issue is that you need to add the portable HTTP client to BOTH your portable class library AND any app that consumes that assembly. I resolved the issue simply by adding the portable HTTP client Nuget package to my app.