I just updated VS2017 to the latest version 15.7 which include Xamarin.Form 3.0. Now I created Blank xamarin form app with net standard to be selected. Open 'MainPage.xaml.cs', InitializeComponent line is in red that indicate "Cannot resolve symbol InitializeComponent". I can build the whole solution successfully.
If I add any UI element in the 'MainPage.xaml', these elements referenced in the code behind file displayed in red. I still can build but just feel not confitable seeing these red error.
I go to "Tools -> Extensions and Updates..." from VS2017 menu, and disable "JetBrains Resharper ..." and then restart VS2017, this issue is disapeared. I believe we may have to wait fix from Resharper in next release after the end of June.
Original post is at https://forums.xamarin.com/discussion/comment/330135