Android.Content and Android.Graphics missing an assembly reference?

I have just update Xamarin VS and have a project which has not changed since Oct 2015, and has been successfully compiling and happy all this time. I am now getting two errors when trying to compile the project.

Error   6   The type or namespace name 'Content' does not exist in the namespace 'Xamarin.Android' (are you missing an assembly reference?) e:\Workspaces\Code Examples\Xamarin\Xamarin.Auth-master\src\Xamarin.Auth\AccountStore.cs    44  46  Xamarin.Auth.Android
Error   7   The type or namespace name 'Graphics' does not exist in the namespace 'Xamarin.Android' (are you missing an assembly reference?)    E:\Workspaces\Code Examples\Xamarin\Xamarin.Auth-master\src\Xamarin.Auth.Android\WebAuthenticatorActivity.cs    165 74  Xamarin.Auth.Android

I have tried clean, build, deleting the bin folder, rebuild, creating a new empty project and adding all the files and still get errors. If I type Android. - Intellisense only gives me the option for Android.Net and nothing else. I can correct the errors by adding Using Android.Content at the top instead of using the full name of
public static AccountStore Create (Android.Content.Context context)

And
public override void OnPageStarted (WebView view, string url, Android.Graphics.Bitmap favicon)

Have I missed a change, or do I need to do something more than a clean and rebuild?